The TARDIS materialises on the beach of a small island. In the centre is an enormous narrow stone pyramid. Inside the ship, the Doctor is pleased with the low radiation count. He activates the scanner, which shows a small stretch of beach and the calm sea. Barbara chides the Doctor for not having "colour television" on the scanner, to which the Doctor confidently replies he does, it's "temporarily" out of order. Barbara and Susan express interest in exploring the seaside, but Ian is concerned. It is clearly not Earth out there. The Doctor feels it is fine to explore as there are no signs of danger about, but Ian thinks he saw some movement on the scanner. He admits he may just be seeing things and so agrees to go outside.

Elsewhere along the beach, four torpedo-like submersibles approach the island through the sea. Three wash up together while the fourth lands separately some way apart. These did not drift here; they travelled and landed deliberately.

Going outside, the time travellers find the sea on one side and tall jagged rock formations on the other. All are pleased at arriving at the seaside, but it is an unearthly sea - perfectly calm and flat. Ian thinks it could be frozen but the Doctor disagrees. It's far too warm here for that. Susan would like to have a paddle in the water, but the Doctor points out that they don't know what kinds of creatures may live in it. The group moves off into the rocks, which become more dense as they go. From behind one of the outcrops, the group is being watched by a strange creature in a black rubber wetsuit, complete with flippers for feet.

Barbara and Ian note the silence and barrenness of the place. It is still and oppressively quiet. The Doctor joins them, showing a chuck of raw glass he has picked up. Could the sand have turned to glass or has the glass - all around them in jagged chunks - been put here deliberately?

As they speculate, Barbara goes to join Susan, who has removed her shoes to take a dip in a small pool she has found. Barbara trips over a rock as she goes and stumbles into Susan. One of her shoes falls in the pool. Susan starts to stick her foot in to retrieve the shoe when Ian arrives and stops her with a shout. He has noticed that her shoe has started smoking and dissolving in the pool. It is not water at all but acid; a particularly strong acid that ate the shoe in no time.

Susan is frightened by this near-miss, but Barbara calms her. To give her something to do, Barbara sends the girl back to the ship to fetch another pair of shoes. Ian lends her his boots so that she won't cut her feet on the glass. She is soon off. Once Susan is gone, Barbara shares an observation with Ian. This acid pool is a tidal pool, meaning that the entire body of "water" surrounding them is acid as well. It ties in with the glass beach.

Back at the TARDIS, another of the wet-suited creatures examines the doors, taking particular interest in the lock. However, it must break off and hide when Susan approaches. She does not see the creature and goes straight into the ship.

The Doctor is astonished by the revelation of the acid sea. He has never heard of anything like it before. He is glad to know that Susan wasn't harmed, but he chides Ian for not wearing shoes. He could have lent them to Susan to return to the ship! Ian and Barbara share a laugh at this and start to correct him, but they are distracted by the strange sight before them: three of the torpedo-like submersibles washed up on shore. They go to investigate.

The tubes are hollow and clear, made of glass designed to traverse the acid sea. Ian thinks they are one-man submarines, all empty. Whatever they are, they clearly indicate that this place is inhabited. While Ian and the Doctor try to unscrew the nosecone opening from one of the tubes, Barbara wanders further up the beach, where she finds the fourth submersible. Only this one is not empty! She shouts out and Ian and the Doctor hurry to investigate.

Meanwhile, Susan has found another pair of shoes and emerges from the ship carrying Ian's boots. As her gaze travels to the sand, she notices strange footprints there. They are flipper-like. She calls out for her grandfather but hears no response. Her curiosity getting the better of her, Susan drops Ian's boots and follows the footprints away from the beach.

From behind a rock, one of the wet-suited creatures emerges. It is fully revealed now to be humanoid only in certain aspects. While it possesses two arms, two legs, and a trunk like a human, its feet are flippers and its head is flat on top and wedge-shaped with an odd geometric antenna protruding from it. It follows after Susan.

Examining the errant submersible, the Doctor finds a crack in it. Clearly the acid seeped into it as it travelled and killed the occupant. Inside are its remains. Ian forces open the nosecone and uses the Doctor's walking stick to fish out the "remains". But all that is left is a protective wetsuit, like the ones that divers wear. Aside from the odd-looking head, the creature who wore it was humanoid. Barbara spots a tear in the material through which they speculate that acid got in. The person was eaten away and only the suit remains.

The Doctor has seen enough, growing concerned over Susan's prolonged absence. He decides to return to the TARDIS. Their progress is halted, however, when Ian spots the top of the narrow pyramid rising over the tops of the rocks. It is extraordinary. The Doctor agrees and hopes to find out there who uses the glass submersibles. After they return to the TARDIS and find Susan, they can do a little "visiting".

Susan has already found her way to the pyramid, arriving near one of the corners. Its base is very large on all sides and blank walls stretch out far behind her and above her. It is made of stone slabs, but there seems to be no windows or doors. She starts moving toward the corner to try another face, unaware that one of the wet-suited creatures awaits her around the turn, a knife in its hand raised to strike.

Meanwhile, Barbara has completed a search of the TARDIS and found no sign of Susan. Noticing Ian's boots lying outside in the sand, Barbara also sees Susan's footprints going off into the distance. The Doctor is not pleased by this turn of events. He has decided that the acid sea and the glass on the beach is part of a defense barrier to keep stranger out. Ian agrees. He finishes putting on his boots and suggests they head for the pyramid structure. Susan's footprints seem to lead in that direction. All three head off.

Susan hesitates before turning the corner, torn whether or not to return to the TARDIS. Her hesitation saves her life. The creature waiting for her around the corner is surprised when part of the wall against which he leans gives way. It opens in revolving-door fashion and the creature falls into the darkness of the building without a sound.

Susan continues, turning the corner. She is unaware of the danger she avoided - no one is there and the entire face is blank and featureless like the last. She continues on.

Ian, Barbara, and the Doctor finally arrive at the building on another face. It is quite enormous from this angle. Barbara is quick to notice the construction of the place - stone blocks perfectly placed with no mortar between. They were placed with tremendous precision, like the pyramids of the Egyptians and the Incas. The Doctor is more concerned with finding Susan and Ian suggests making a circuit of the building. The Doctor agrees, sending them one way while he goes the other. They should meet up again at the far corner.

Susan continues her own circuit on a different face, but she does not get far before a hidden revolving door swings round when she leans on it. She falls into the building with a scream

Ian and Barbara hear this and recognise it immediately. They hurry toward the corner.

The same fate befalls the Doctor and soon he is inside as well.

Ian and Barbara turn the corner cautiously but see nothing. They both swear they heard her. What's more, there's no sign of the Doctor either. He should have reached the far corner they can see in the distance by now. Their party is being split up.

Inside the pyramid, Susan wanders down dark, dank corridors slowly and as quietly as she can. She has seen no one so far. However, the wet-suited creature is also free inside and hides in the shadows waiting for her. Susan is startled by the appearance of a man in a white monk's robe and hood. She cannot make out his features and he does not speak. He turns and gazes at her - and past her - for a moment and then leaves down a side corridor as slowly and quietly as he came. Susan's fear grows and she backs cautiously down the corridor.

However, this takes her right into the clutches of the wet-suited creature, who grabs her. Susan screams and struggles, pushing the creature back against the wall. Suddenly the creature stiffens and lets her go. It topples over, dead, with a knife in its back which must have come through the wall somehow. Then the hooded man reappears, bearing down on her. This is all too much for Susan. There is nowhere left for to retreat.

Outside, Barbara rests on the ground, trying to figure out their next move. She suggests another circuit of the walls, but Ian thinks that futile. He goes round the corner, still looking for the door. Barbara is concerned when he does not respond to her next suggestion and goes looking for him, panic setting in. To her horror, there is no sign of Ian. She leans against the wall to steady herself and it gives way. She falls through with a scream. Waiting for her is the man in the monk's habit.

Ian is alone inside the corridors. He finds the body of the creature in the wetsuit who was killed in front of Susan. He is at a loss as to what sort of place they've landed in. He hurries further into the building.

The Doctor, Barbara, and Susan are all fine, held prisoner but together. Each is relating his or her own odyssey to reach this place. Susan tells Barbara of the creature she saw killed. It sounds to the Doctor just like the wet-suited creature they discovered earlier. He has determined that the man in the monk's habit is the one who inhabits this building. The wet-suited creatures are invaders here, just like them. But the Doctor points out one puzzling - but comforting - fact. The creatures have been killed while they are merely prisoners. Barbara is concerned that they still might be killed, but the Doctor pins his hope on Ian. Whilst he's free, they still have a chance of escape. Barbara is sad to say that Ian has already been "captured".

Elsewhere in the corridors, the man in the monk's habit is set upon by the second of the wet-suited creatures. It has a knife and the man must struggle for his life. But he is old and loses ground quickly. Ian happens upon the scene and joins in the fray, just as the man is losing. He manages to disarm the creature and finally to shove it hard against the wall. The fake stone panel gives a little and the creature totters unsteadily. When the man in the habit pulls a lever, though, the panel gives way completely and the creature plummets to his death in an acid pool far below.

The man in the habit is revealed to be an ordinary man, old and worn. He is tired after his struggle. Ian assumes he is a prisoner her, held captive by the creatures. The man, called Arbitan, assures Ian that while he can never leave here, it is actually his home. Ian inquires about his friends, but Arbitan's thoughts and words are scattered. He saw the TARDIS materialise and he thought they were hostile. The Voord - the wet-suited creatures - have been trying again to invade this place after a long absence. He alone defends it and he is clearly weakening. He can see Ian's confusion and, catching his breath, offers to release Ian's friends. Then, he promises he will explain everything. The two men head off, unaware that they are being watched closely by the third Voord.

Soon the Doctor, Susan, and Barbara join them in the main control area and Arbitan sets about explaining. The room is dominated by a giant machine with a five-sided control console in the centre. Arbitan calls it the conscience machine and says it was developed 2000 years ago as a totally fair and impartial judge and jury. Then its power was expanded to radiate out over the surface of Marinus - this planet - and to eliminate evil, fear, and hate from the minds of all people. For 700 years the planet lived in complete peace.

But then a man called Yartek developed a means to resist the control. He and his followers, the Voord - were able to rob, kill, and control others with impunity. Ian is astonished, realising that the conscience machine would be an irresistible target for the Voord. With it they could control all of Marinus. Arbitan tells him that this was foreseen. But, instead of destroying the machine, they disabled it by removing the 5 microkeys which control it. One is kept here with him and the other four were hidden around the world in locations known only to him. The time has finally come when the machine has been modified sufficiently to control the Voord again. The keys cannot be reconstructed, only found and returned. Over the years, he has sent all his friends and followers - even his own daughter - to find the keys. None have returned. Now he is all alone here, fighting off the Voord attempts to capture the machine. But Arbitan's melancholy gives way to sudden hope when he realises that he now has new searchers right in front of him: the four time travellers!

Some time later, the 4 travellers traipse back to the beach toward the TARDIS. They have refused Arbitan's request and - somewhat sadly - have decided to leave the planet. However, when the Doctor tries to approach and unlock the doors, he is stopped well short by some sort of invisible barrier. A bit of investigation reveals a circular force field all around the ship with no weaknesses and no way in. Arbitan's voice booms out over the beach. He tells them he had no alternative but to force them to agree. If they find the keys of Marinus and return them to him, he will let them go. If not, they will be refused food and water and will be forced to stay here until they agree. It is clear that they have no choice.

Back inside the pyramid, Arbitan explains the geographical locations of the four keys. He promises them the force field will be released as soon as they've left here for the first location. The TARDIS will be available to them as soon as they return. The Doctor is ready to protest Arbitan's outrageous actions again, but Ian urges him to simply get on with the job. The sooner they start, the sooner they'll finish. Arbitan also asks them to search for his daughter along the way.

The Doctor is concerned that they will be asked to travel by primitive submersible, but Arbitan presents them with quite a sophisticated alternative: spatial travel dials. They are wristwatch-type devices which work as the TARDIS does - through dematerialisation - but can travel only through space. The entire journey is preprogrammed into each unit. The Doctor is pleased at this. Barbara - testing the dial as Arbitan instructs - suddenly vanishes, already on her way to their first destination. Susan and Ian are worried that something may happen to her, but Arbitan assures them the travel dials are safe. They all must follow quickly, but he leaves them with one final warning: if they return and find the Voord in control of the building, they must destroy the keys rather than handing them to the Voord. With that, the three twist their dials and vanish.

Arbitan bids them success for the sake of all his people. He walks slowly over to the conscience machine. So engrossed is he in his thoughts that he does not see or hear the third Voord enter. The creature is easily able to sneak up on his enemy and stab him in the back. Arbitan falls to the floor.

The Doctor is certain Barbara would not relinquish the dial by choice, so something must have happened to her. Behind them is a huge door, the only place to begin searching for answers. The Doctor councils caution, but Ian forces the doors open. They stumble inside only to be assaulted by a blinding light and a piercing alarm-like noise. All 3 cover their eyes and ears, moving further inside

The assault ends and they find themselves in a paradise - a room very like one in ancient Greece, full of statuary and ornate wall coverings. It is decadent and beautiful. Here they find Barbara, dressed in beautiful silken robes and lounging on a chaise. A pair of female servants stand by. Susan runs to her, glad that she is unharmed. Barbara explains that when she turned the travel dial, she got scared at the sensation of falling and tried to tear the dial off her wrist. He scratched herself in the process, leaving blood on the dial.

Ian admires the surroundings and the central part Barbara seems to have in it. She responds by sending the serving girls to fetch more food. Ian admires the women too as they go. He and Susan attack the sumptuous fruit in the bowl next to Barbara. The Doctor pronounces the place sensuous and decadent, but not unpleasant, before taking a small pomegranate from the bowl. Susan notices a bolt of fine silk lying on one of the other chaises and goes to examine it. Barbara tells her it was brought to her to approve for her robes. Susan, too, would like a dress made from it.

Shortly the serving girls return, with two others, bearing a feast of roasted bird, vegetables and desserts. Barbara and Susan tuck in hungrily, and even the Doctor is tempted by a truffle. But Ian refuses to take anything. He "hasn't seen the menu yet" and doesn't know the price of all this "hospitality". Barbara says she's met their host and he seems a perfectly fine man. It is not long before he returns, a handsome young man called Altos. He says he is merely here to see if they want anything else and says nothing of the "price". To satisfy Ian, Barbara asks for information on this place.

Altos tells them they are in the city of Morphoton, a place where every person's every desire is granted, creating a peaceful and contented society. Against the Doctor's wishes, Susan pipes up with a request for a silk dress. Altos assures the Doctor her request is no imposition at all; he is here to serve. She shall have the dress she wants. He also tells the Doctor he will have what he wants: a state-of-the-art laboratory with every conceivable instrument. Ian is taken aback at the ease with which this wish was granted. Altos promises them all that they will see and appreciate Morphoton for the haven that it is when they see the laboratory and the dress in the morning. With that, Altos bids them goodnight.

The Doctor is fascinated by this society and thinks a study of it will be most interesting. But for now he wants only sleep and settles quickly on one of the chaises around the room. Barbara helps a suddenly somnolent Susan to bed as well. But Ian is still concerned with a very real question: how rich and powerful does one have to be to give all these things away for free. He has also noted that Altos didn't blink at all when he was with them, but thinks he may be imagining things. Barbara warns him not to judge this society by Earth standards and tells him he is misinterpreting these kind and hospitable people. Perhaps he'll feel differently in the morning. Ian heads for bed and is soon asleep. Barbara, too, lies down.

Shortly, the eyes of a wall-hanging sculpture light up and a soothing heartbeat and breathing sound fills the room. A secret door slides open and a serving girl enters silently. She moves round the room, placing small plastic disks on the foreheads of the sleepers. Then she is gone. But Barbara is not asleep. Her head turns and the disk falls off her head. She sits up, intent on retrieving it, but is suddenly assailed by the blinding light and the piercing noise. They attack her brain and eventually knock her unconscious. As she slumps onto her bed, none of the others react at all.

The next morning, Ian, Susan, and the Doctor enjoy a full breakfast, accompanied by a seemingly endless supply of fresh juice. All are chipper and happy. Barbara, however, is still unconscious - asleep, they believe. They decide to let her lie in a while longer. The Doctor notices Ian scratching at his forehead and complains of a sore spot on his as well. They can see nothing so think nothing more of it.

Shortly, a serving girl arrives bearing Susan's silk dress. She is very pleased with it and is anxious to show Barbara. The Doctor allows Susan to wake her up. However, when Barbara wakes, she is horrified to see that in place of the sumptuous room they were in before, there is now a dank cellar covered in ripped, dirty cloth festooned with spider webs and dirt. But she is the only one who can see it. From their perspective, the others see the room as it was yesterday. They are very concerned for Barbara, thinking she is dreaming or hallucinating. When the Doctor offers her fruit juice in a cut glass goblet, she sees only dirty water in an unwashed old mug. She slaps it from the Doctor's hand and breaks it, growing more desperate by the moment when she realises she is the only one who can see these things.

Barbara tries to tell them of the ruined furniture and the raggedy dress she wears, but they cannot see it. They believe she is hallucinating and she believes they have been blinded to the truth. When Susan shows her the new silk dress, Barbara sees only dirty rags. She begs her friends to "try to see the truth", but they only question her mental stability. For her part, Barbara is convinced the "generous" people of Morphoton have done something to their minds to blind them to the reality around them and she vows to make them see things how they really are.

When Altos enters, Barbara can see in his eyes that he knows what's happened. He knows that the conditioning has failed on her. She becomes frightened. Altos speaks gently to her, expressing concern that she may be ill. Barbara resists when he takes her arm, saying he will take her to their physicians. Her struggles become more insistent and she breaks free of his grip, running from the room. At a loss to explain her behaviour, Ian offers to help find her and bring her back. Altos refuses this offer, telling Ian to leave everything to him.

Barbara races to the first unlocked door she finds. It leads down a short staircase to a small, dank, featureless room. The sound of Altos' footsteps reaches her and she hides behind a column protruding from the doorway wall. Altos' search is brief and cursory, thinking she is already far ahead of him. He does not see her hiding there and so leaves, closing the door behind him. Barbara returns to the door, ready to leave and return to her friends, only to find the door now firmly locked from the outside. She is trapped. Overwhelmed with confusion, sadness, and fear, she sinks to the ground, near tears.

Altos, unable to locate Barbara, heads for the hears of the city - the room in which live the true masters of Morphoton. The room is dark and windowless. Along one wall is a bank of instruments. A steady heartbeat-like sound issues from all around and a serving girl waits nearby, staring blankly ahead until needed. And in the centre...the masters. Four hideous brain-creatures sit atop a table, each in a tall glass bell jar. Each is a sickly pink colour with pulsating tendrils. Aside from two bulbous eye stalks, they have no other physical features. The disembodied creatures speak through machines, one deep authoritative voice speaking for them all.

Altos reports what has happened: Barbara has resisted the power of the mesmerent that blinds and controls everyone else here. She has escaped and cannot be found. The brain creatures are angry and concerned. The serving girl who placed the disks on the travellers' foreheads - Sabetha - must be imprisoned and punished for her failure. Altos is to make up a story about Barbara to reassure and distract her friends. After 1 more exposure to the mesmerent, they will be completely subjugated like all the others here. As for Barbara, she has seen the truth, the ruined society masked as a place of decadence and plenty. She cannot be controlled. She must be destroyed.

Barbara sits silently in her makeshift cell, almost despairing of escape. She is startled by approaching footsteps and hides again. Another prisoner is shoved in and the door shut firm again. The newcomer is a blonde-haired serving girl, dressed in rags like the others. She is in deep shock and gazes ahead of herself blankly. Barbara recognizes her as the woman who placed the disk on her forehead and addresses her. She tries to pump her for information but the girl is focused only on the fact that she is to be punished. The girl, Sabetha, is very frightened. It will take much effort for Barbara to break through her conditioning.

Altos tells Ian and the Doctor that Barbara is suffering from a nervous condition and that she is under the care of a physician. She is deeply sedated but will recover. He tells them this as they walk along a corridor. Both Ian and the Doctor seem convinced of his sincerity and are now anxious to see the promised laboratory.

Altos opens the door and leads them inside. In reality, it is a blank white room with a bench against one wall. Only a dirty, chipped mug sits on it. In the eyes of the Doctor and Ian, having been exposed to two doses of the mesmerent, it is a laboratory filled with scientific instruments including a cyclotron. Both are fascinated and impressed. The Doctor picks up the mug, seeing instead an advanced instrument that may help him repair the time control on the TARDIS. Both are convinced that the miracle of Morphoton is true.

Sabetha has retreated inside herself, absently stroking a pendant around her neck. Barbara is stunned to see that it is the first microkey for which they are searching. Sabetha keeps it from her, saying it was given to her as the thing she desired most. Barbara learns that she was sent by Arbitan to retrieve the key, but Sabetha's mind is so clouded that she cannot remember anything more. Barbara is convinced that this is Arbitan's own daughter.

In the control room, the brain creatures are preparing the final stage of processing on Ian, Susan, and the Doctor. Altos himself is placing the somnar disks this time. Soon their will, their memories, their selves will be gone, enslaved to the masters of Morphoton. Altos returns for orders. Once the travellers recover, Ian will join the slave labour parties while the Doctor will be but to work in a research project and Susan will be trained to serve. She will replace Sabetha. The search for Barbara will resume in the morning. She must be found and destroyed.

In her cell, Barbara as been trying all night to get more information from Sabetha, but the effort to remember has been too much for the girl and she is very tired. They are interrupted by the arrival of Altos. He is here to fetch Sabetha. Barbara tries to sneak out the open door behind him, but he hears her footsteps on the stairs and turns to find her. Altos grabs her and they struggle. He gains he upper hand and begins to choke her, to kill her as his masters have ordered. But Sabetha has regained enough of herself to see what is happening. She smashes a clay pot over Altos' head, knocking him out. Barbara races out to try and find her friends, vowing to return for Sabetha. The weary girl does not even acknowledge her and slumps to the floor, exhausted.

Barbara makes her way through the corridors carefully. She cannot believe her luck when she finds Ian. She runs to him, relieved. However, her happiness is short-lived when she realises that Ian has been as thoroughly hypnotised as Sabetha has. He does not recognise her, but knows that she is an escaped fugitive who must be captured. Ian takes her to the central control room. She is weary and dejected, but when she sees the repellent brain creatures, her fight returns. She tries to get Ian to see what is happening, but he only holds her, staring blankly.

The creatures tell her that they are in control here. Their brains outgrew their bodies and their intelligence created this city to serve them. The humans here are their machines to feed and tend them and do their will. The illusion of paradise snares them in. Barbara thinks she too is to become a slave, but the creatures tell her they cannot erase the truth from her mind once she's seen it. They order Ian to kill her.

Blankly, Ian starts to choke his friend, egged on by the insistent voice of the brain creatures. But Ian is too much of a slave, moving and acting mechanically. Barbara is easily able to counter his half-hearted force and break free. She begins attacking the machinery, then the creatures' jars. Their voice turns to a scream as she smashes it all. Without their technology and life support, the creatures quickly die. Ian recovers himself immediately and runs to Barbara, not sure where he is or how he got there. She reassures him that all is well at last.

Later, the Doctor, Ian, and Barbara arrive at a hillside dwelling overlooking the city. From its terrace, they can see the now-freed citizens burning their city. The Doctor can hardly blame them, having seen its true nature for the first time. They await Susan, who is bringing Sabetha and Altos with her. It seems he too was sent by Arbitan some time ago. Both of them will join the search again, but the Doctor has a change in plan. He produces all of their travel dials. Luckily, the brain creatures kept them safe, unaware of their significance.

Susan and the others arrive. Altos explains that he and a friend called Eprim were sent together by Arbitan. Eprim went ahead to the civilised city of Millennius to find key four, while Altos searched for the other three. Something must have happened to him there, or perhaps he is simply still searching for the key. The Doctor tells his startled friends that he is going to adopt this plan, going ahead alone to Millennius to make contact with Eprim if possible and/or find the 4th key. The others will continue their search for key 2 together. Susan protests, but the Doctor explains that this is the best and most expedient way. Reluctantly, Susan agrees. They settle on seven days to meet again in Millennius.

All of them put on their travel dials and prepare to go. Susan, not wanting to face a sad goodbye, hastily activates her dial and vanishes. The Doctor urges the others to follow her quickly.

Ian, Barbara, Altos and Sabetha arrive and go immediately to help Susan. They cannot understand why she is in pain as they can see and hear nothing unusual. Susan, too, can hear nothing now. She stands up, still very frightened and unable to articulate what she heard. Barbara comforts her, believing she will tell them in due time what happened. Susan thinks they don't believe her.

Ian, Altos, and Sabetha explore a bit. A high stone wall encloses them on two sides. It is completely overrun with vegetation. Sabetha calls the place "dead" even though there is an abundance of plant life. Ian locates an archway in the wall but thinks it impassable due to the thick, tangled creepers which cover it. He decides to make a tour of the walls with Altos and Sabetha to try and locate another entrance. Barbara decides it is best for the frightened Susan - who is also worried about her absent grandfather - if they stay put. Barbara plans to look for strong branches to use to force through the archway if necessary. Ian agrees, but warns her not to try and go in until he and the others return.

When Ian is out of earshot, Barbara complains half-heartedly about Ian treating them like fragile china, but Susan is glad that he looks after them. Barbara grudgingly agrees, but admits she likes to rebel every once in a while. Susan is now feeling better and tries to describe for Barbara the terrifying sound she heard earlier. She says she recognises it but cannot recall how. She only knows that it is something evil.

Barbara tries to take her mind off this by exploring the area. She, too, is amazed at the aggressive growth of the plants. They will soon crumble the walls. Susan sits on the leaf-covered ground as she and Barbara talk, unaware of the creeper that is now trailing down the wall and actively moving toward her. Barbara examines the archway carefully and realises that it isn't nearly as densely-blocked as Ian suggested. She is sure they could get through, but is interrupted by Susan's scream before testing the theory.

The creeper has entangled her ankles and she is sure it is trying to wrap her up. Barbara rushes to her and pulls off the vine, smashing it with a rock. She tries to tell the hysterical Susan that the vine could not have deliberately attacked her, but is hard-pressed to convince her. Barbara's mind moves back to the problem of the archway and she encourages Susan to help her. Together they create an opening large enough for Barbara to get through. As she does so, she sees another wall ahead, featureless but for a large, grotesque golden statue. While Susan calls after her, Barbara moves forward to investigate.

Ian returns, having had no luck in finding another entrance. He is angry that Barbara has ventured through the archway alone. Rebelling for a moment, Barbara tells him not to worry. She's just exploring a bit. The statue is of a sitting humanoid, very tall, with the trunk of a man and the stylised head of a lion bearing a crown of sun rays. The arms are outstretched and look different from the rest of the statue. As Barbara looks it over, she cannot believe her luck. There, on top of the lion's head, is the second microkey. Hurriedly, she reaches for it.

Altos and Sabetha return and learn the good news. All are overjoyed that this part of the quest has been completed so easily. To reach the key, Barbara must climb upon the statue's base into its outstretched arms. She gets the key, but not before the statue's arms close around her legs, trapping her. She calls for help, but Ian and Altos cannot get through the foliage fast enough. By the time they reach the wall, the statue has rotated on its base, bearing Barbara to the other side of the wall.

There appears to be no other way to get through and no way to bring the statue back. Frustrated and angry, Ian leaves the small yard and heads back through the archway where he can think more clearly. Altos follows, but not before finding and picking up the microkey Barbara dropped in her struggle with the statue. Ian tells Sabetha and Susan what has happened. He is deeply worried about Barbara and barely acknowledges Altos and the key. Altos gives the key to Sabetha for safekeeping.

Sabetha suggests that Barbara may have used her travel dial to escape. As long as she wasn't injured, she could use it quite easily to get away to their next destination. Ian finds some hope in this, but worries that she hasn't had the opportunity to activate her dial yet. The sound of the statue rotating reaches them and through the foliage they can see that it has returned, empty-handed. Sabetha prefers to think Barbara has used her travel dial and escaped, but Ian still worries that she was injured...or worse. But to cover all possibilities, he decides to send Altos, Sabetha, and Susan ahead where they can be with Barbara if she's there. He himself will stay and get himself trapped by the statue to see if he can find any sign of her. Once he's convinced she's gone, he will jump ahead to join them.

Susan reluctantly agrees to go with Altos and Sabetha, worried as she'll now be cut off from all of her friends. But Altos promises to look after her. They activate their travel dials, but Sabetha hesitates when she nearly drops the second key. She puts it on her chain for safekeeping, but in doing so she notices that the second key is slightly smaller than the first. It is a fake; she is certain of it. Ian grows angry, realising that they are now no further along than when they first arrived. Sabetha offers to stay and help him search, but Ian knows the others will be worried if she doesn't arrive. He must search for the true key - and Barbara - alone.

Sabetha vanishes and Ian hurries to the statue, impatient to get on with the search. He climbs up on the base and the arms close around him. The base rotates and he finds himself in an inner courtyard. Released by the statue, he moves to look around. He finds no sign of Barbara, but more of the thick vegetation and more statues of different types. One is an armoured knight or woodsman wielding a long and lethal-looking axe. Ian does not notice as he passes it that his steps have activated a hidden switch. The axe raises and prepares to come down on his skull.

A timely shout from Barbara alerts Ian and he dives away to safety. The axe lands harmlessly in the ground. Barbara comes down from the landing where she was and joins Ian, making sure he's OK. She has been waiting for him, wondering whether or not to use her travel dial. Ian tells her of the fake key and that the real one must be in here somewhere. Barbara is sorry to hear this as she has found the courtyard full of booby traps like the axe. She herself barely escaped being crushed by falling stones. Ian agrees they must go carefully.

Ian spies the landing where Barbara was earlier. It leads to a doorway into the ramshackle building. It's as good a place as any to start. The vines are thick here as well. They seem to weigh on the building, pressing in as if to try and break in. It is everywhere and very worrying. Ian tries to open the door, but despite its crumbling appearance it is solidly locked. Barbara remembers seeing some iron bars in a nearby alcove and Ian goes to find them to help in breaking down the door. Barbara waits for him, unaware of the creeper trailing down behind her.

Inside the building, a withered old man in a dusty robe seems to be spying outward. After a moment, he goes into an anteroom, shuffling slowly and unsteadily.

Outside, the door behind Barbara opens of its own accord, moving on squeaky hinges. The creeper retreats. Barbara calls to Ian, but does not tell him what is happening. He continues looking for the appropriate tool and says he'll be with her shortly. Barbara takes a few experimental steps into the anteroom. It is featureless and empty. However, before she can turn and go back, a net falls from concealment in the ceiling and covers her. She screams as she falls to the floor. But her problems are just beginning. Looking up to the ceiling, she sees a panel embedded with razor-sharp knives. Suddenly it begins to descend toward her. She screams even more loudly.

Ian hears this scream and grabs one of the iron bars he's found, ready to go to her rescue. However, the bar is just another booby trap. A chain attached to it causes a set of metal bars to slide down and close off the alcove. He hurries to find a pick with which to attack the bars.

Barbara struggles and calls out for Ian as the knives descend ever closer. She tries to flatten herself as close to the ground as possible, not noticing that the disheveled old man has appeared in the inner doorway and now watches her struggles. Just as the knives reach her, the panel stops moving. A second later, it is moving jerkily upward and Barbara relaxes, glad to be alive.

The old man now stands over her and peppers the helpless woman with questions. He wants to know why she has come in search of the key. He refuses to free her from the net, fearing that she may be a Voord, even though she doesn't look like one. He does not believe her story that Arbitan sent her, but she shows him her travel dial as proof. The old man takes it from her. If it is set correctly for the entire journey, then he will believe her. Barbara struggles partially free of the net and asks the old man what happened to Ian. He does not answer but instead disappears into the building, closing the door behind him.

Ian finally manages to pry loose the bars and races for the building. He finds Barbara safe but just freeing herself completely from a net. Inside, the old man suddenly calls for help in a choked voice. He has been attacked by a creeper that has wound round his throat and now threatens to choke the life out of him. Ian and Barbara run to his aid but cannot loosen the vine. Ian spots an old sword nearby and finally manages to hack the creeper apart. They help the old man to lie down. He looks decidedly worse for wear. With wild eyes and a rasping voice, the man tells them "it is starting again". When the whispering starts, he says, then comes death. Then he passes out.

Some time later, the man awakes. Barbara has been tending to him and she thinks he is dying. Ian desperately tries to learn the location of the real key, but the old man rambles. He explains the false key and the traps he set to stop the key falling into Voord hands. With that, he slumps back, his energy ebbing quickly. He knows he is dying and seems to want to be reassured that he has complied with Arbitan's wishes. Barbara watches, unable to help, as the man gathers the last of his strength. He manages to point to a doorway and tell Ian "D-E-3-O-2". He adds a warning about whispering, darkness, and danger. Then he is dead, leaving Ian and Barbara little wiser.

After a moment of respectful silence, they head to the inner room - the direction the man pointed - to try and find the key. They find themselves in what looks to be a botany lab, benches littered with potted plants, jars, phials, notes, and dusty books. Ian thinks the letters and numbers could be the combination to a safe where the key is hidden and is overjoyed to see a safe sitting on one bench. However, two attempts to open it fail. Perhaps their first thought was wrong - the man was one for traps and decoys. They decide to search every inch of the room for the key. If they do not find it elsewhere, then it must be in the safe.

Quite some time later, they are still searching with no luck. Barbara wonders what the old man meant about darkness and whispering. She wonders if it has any connection to the noises Susan heard when she first arrived. Ian thinks it doubtful, but finds some clues in the journal of the old man's experiments. In it, he writes of "increased destructive power" and accelerating the "tempo of destruction". While Barbara agrees it's fascinating, this speculation is getting them no nearer the key. She looks in another book. However, she and Ian shortly become aware of a strange sound all around them which now grows in intensity. It sounds like distant anguished whispering with a distinct edge of menace, similar to what Susan described. Barbara has noticed that night has fallen outside and remembers the old man's warning about darkness. To her it sounds like something trying to get in. With rising horror, Barbara and Ian realise it is the plants making that noise as they try to force their way inside the building.

Suddenly a large tendril bursts through the wall and everything is clear. The old man created a growth accelerator which has affected all the plants. The destruction they would normally accomplish in 100 years will now be complete in just a few minutes. And Ian and Barbara are in their path! More tendrils burst in, smashing jars and equipment. A tendril coils itself around Barbara's ankle and she cannot get away. She screams and Ian hurries to untangle her. The vine fights back but soon she is free. How will they ever find the key in this chaos.

However, the chaos provides the answer when Ian notices the label on a jar lying smashed on the floor. The letters and numbers the old man mentioned were a chemical formula -- DE₃O₂. The key is hidden in one of the jars in the lab. They split up and search different parts of the room, dodging both the plants and the debris they generate. It is Barbara who finds the right jar, a dark mineral substance labelled DE₃O₂. Emptying it, she finds the key and rejoins Ian.

Ian tries to help Barbara up, knowing they will both freeze to death if they don't keep moving. But the biting wind and blowing snow has already taken its toll. Barbara only wants to sleep. She stretches out in the snow and closes her eyes, oblivious now to the numbing cold. Ian, too, succumbs, passing out beside her. Barbara's eyes open momentarily and she sees what she thinks must be a hallucination - a burly, bearded man dressed in furs looming over her. Then the cold takes her and she too passes out.

But this was not an illusion. The man looks them both over, his gaze drawn by the object in Barbara's hand. He takes the precious microkey from her.

A while later, the man returns to his home. He is a trapper by trade, living in a small rustic cabin with no windows and a heavy wooden door. Furs line the walls to keep out the cold. Outside, the wind howls and inside a fire blazes. He locks the door and then goes to tend to his guest: Barbara. He sits down next to her and awakens the sleeping woman. She is disorientated but alive and well. She is relieved to find that Ian is here too, still unconscious on a couch.

The man ignores Barbara's questions and takes her hand in both of his. He sees signs of mild frostbite and begins massaging her hand, explaining that this will restore circulation and warmth. Despite this outwardly caring gesture, the man has a glint in his eye that makes Barbara nervous. He asks her if she is afraid of him. When she lies and says no, he tells her that he once killed a wolf with his bare hands. He introduces himself as Vasor, feared by most men. He rarely gets visitors. Barbara is none too pleased with this or with the attention he is lavishing on her hand and pulls it away. With studied patience, Vasor goes off to fetch warm drinks.

Barbara hurries to Ian's side and wakes him up. His hands are nearly frozen, but he is otherwise unharmed. Barbara massages Ian's hands to restore circulation. Neither of them knows where they are or how they got here.

Vasor returns with bowls of warm liquid which Ian and Barbara down quickly. Vasor tells them he found them on the mountain when he was out setting his traps. Barbara thanks him for saving them from the cold, but Vasor assures her the wolves would have gotten them before the cold. There are more than ever around this winter, he tells them, and they are even so bold as to attack the local villages. But they are 3 miles away from the nearest village, in the mountain wilderness. He is relatively safe from the wolves out here, as are his traps.

Vasor admits that he had help in carrying Ian and Barbara here. A stranger came early last night, raving madly and looking for two girls. Vasor gave him provisions and helped him in his search. Ian and Barbara are certain this stranger is Altos and become excited. Vasor tells them that the stranger went off to the nearest village after finding and bringing Ian and Barbara here. He likely will not be back tonight as the gathering darkness will make it too difficult to traverse the ridge between here and the village. Ian insists on going out to try and find him but Vasor refuses. There is a storm coming.

Ian realises he cannot ask the man to risk his own life to find Altos and so asks to borrow warm clothing. But the trapper is wily - and greedy. His furs are his livelihood and he demands payment for them. He eyes Ian's travel dial and reluctantly Ian hands it over. Vasor goes to fetch a lantern while Ian suits up in a fur cloak. He must hurry if he is to find Altos tonight, to say nothing of Susan and Sabetha. Vasor gives him the lantern and a pack and directs him through the fir forest toward the village. Ian promises to be back soon and heads out into the frozen night.

Barbara crosses to the fire as Vasor locks the door. There is something ominous in the way the bolts snap to. Vasor crosses to Barbara and puts his hand on her shoulder. He seems very pleased to be alone with her. Barbara clings to the hope that Ian will be back soon as Vasor goes to prepare a meal while they wait. His quip about "fattening her up" does not make Barbara feel any safer.

Ian trudges up the mountain, the lantern barely illuminating the way ahead. A wolf bays nearby. He can just see its outline in the distance. Moving forward, Ian nearly stumbles over something in the snow. It is a body. It is Altos. Ian checks him over quickly and finds he is still alive, although suffering from the cold. He is unconscious and badly frozen, and his hands are tightly bound. He did not end up here on his own.

Vasor and Barbara finish their meal in edgy silence. The sound of howling wolves is nearby and Barbara becomes worried. Vasor seems not to have a care in the world. He is certain that his door can hold back the wolves if they should come. He takes two of the dishes and clears them, leaving the rest for Barbara. She picks up the utensils and looks for a place to put them. Opening a drawer in the table, she finds to her astonishment four travel dials and Sabetha's necklace, the three keys still attached.

Vasor catches her snooping and becomes angry. Despite her growing fear, Barbara demands to know how he got the items. He says he found the two girls on the mountain yesterday and gave them food and flint in exchange for their "trinkets". He is not sure whether they are still alive or not. It is not his job, he says, to look after every fool who gets lost on the mountain. He also changes his story about Altos, saying they were looking for the girls when they stumbled across Ian and Barbara. Then Altos wanted to go out again.

But Barbara no longer believes Vasor's stories. She is certain that Sabetha would never willingly part with the keys and accuses Vasor of stealing them. The trapper laughs this off, taking it as confirmation of the value of the keys. Barbara fears that Altos and the girls have been killed, but Vasor laughs again. In rough country like this, one need not kill. The cold and the wolves do that well enough on their own. Barbara vows to make him answer truthfully when Ian returns, but Vasor is certain Ian will not return, not with the "surprise" he put in Ian's bag.

Meanwhile, Ian has succeeded in reviving Altos and both are working to restore circulation to his legs. The wolves are still howling, the sounds seeming much nearer now. Altos tells Ian that he was tied up by Vasor and that he rescued Ian and Barbara only because Altos forced him to. Vasor is a ruthless liar and opportunist. Ian's blood runs cold, realising he has left Barbara alone with the trapper. They must hurry back.

Ian helps Altos to his feet and goes to retrieve his pack. The flap falls open, revealing the contents: raw and bloody meat. When the wolves howl again, even nearer this time, the reason is clear. Ian was not meant to return alive. Ian chucks the pack out into the snow, hoping the wolves will home in on its scent and leave him and Altos alone. They hurry off back to the trapper's hut.

Vasor has dropped his pretence of beneficence completely and tries to catch Barbara. She dodges out of his way and grabs a fireplace poker as a weapon. Vasor hesitates as he is in no hurry. No one is coming to help Barbara. She will tire and he will catch her eventually.

Ian and Altos make their way through the snow, still pursued by the wolves. They are after live prey. Peering through the snow, they catch sight of Vasor's shack about 1/4 of a mile away. They might make it, but they'll have to run. Altos summons his strength and prepares for the attempt.

Vasor's "patience" is short-lived. He attacks Barbara, who jabs at him with the poker. She hurts him, but this only makes him angrier. He chases her round the room, deflecting her every weapon. Shortly she stumbles and falls onto the couch, trapped. Vasor moves in for the kill, but stops when he hears pounding on the door. Ian and Altos call out for Barbara. She dodges around Vasor and reaches the door. Vasor grabs her but she bites his hand and he lets go. Barbara open the door and Ian and Altos burst in. Vasor retreats to a corner.

Barbara tells them about Susan and Sabetha being left in a cave on the mountain and Altos attacks Vasor. The trapper's swagger is gone and he swears he left the girls flint and food. But Altos doesn't care. He is prepared to pound the treacherous Vasor. Ian leaps forward and stops Altos. He wants Vasor unharmed to lead them to the cave.

Susan and Sabetha are indeed still alive, but not for long. They are indeed in a cave, but their food is gone as is their flint. The fire's embers are so low as to be unable to catch fire again, and there is no more wood. Susan and Sabetha cling to each other for warmth. Susan knows they haven't got much time left but refuses to go down without a fight. She says they should go outside and at least try to find some help. Sabetha agrees but thinks that Susan has chosen the wrong passage as the exit. Neither can remember the way they came in. With little choice, they take the passage Susan suggested.

Vasor leads Barbara, Ian, and Altos up the mountain. They are getting closer - Vasor says the cave is just over the next ridge. However, a genuine fear has crept into Vasor and he wishes to go back. Ian refuses and shoves the trapper onward.

Unfortunately, the passage Susan has chosen is a dead end. They are trapped and must go back. Both are getting colder by the minute.

Vasor and his group arrive in the cave, but it is empty. Ian sound ready to do violence to the man, but he catches sight of the burnt-out fire and knows that Vasor was at least telling the truth. The embers are still warm, meaning Susan and Sabetha haven't been gone long. Vasor says this is the only exit, so the girls must have gone deeper into the mountain. But Vasor does not wish to go on. On the verge of hysteria, he says there are demons in the mountain and he dare not go in. Ian refuses to listen to the ridiculous story and pushes him onward insistently.

Susan and Sabetha's path has lead nowhere and Susan thinks they should change direction. But Sabetha wants to press on, thinking the tunnel must lead outside at some point. They arrive at a wide chasm, but it is traversed by a crude rope bridge. First Susan, then Sabetha crosses it. They continue on through the next tunnel.

Elsewhere, Altos leads their party now, with Ian bringing up the rear to keep Vasor from bolting. Barbara notes that the walls of the tunnels are not rock but solid ice. However, Vasor is consumed only with his fear. He tries to protest again about the demons, swearing that people have truly seen them. Ian is unsympathetic, suggesting that Vasor is about to become one of those eyewitnesses himself! A short time later, they too arrive at the rope bridge.

On the other side of the rope bridge, Susan and Sabetha have emerged into an ice cavern. It is tall and open. In the centre is a giant block of ice guarded by four armed, uniformed and helmeted ice soldiers. They appear long dead, frozen in place. Frightened, the girls retreat from the cavern.

Altos and Barbara are already across the bridge when Susan and Sabetha emerge from the tunnel at a run. Their reunion is a joyful surprise. Ian, too, is pleased to see them safe and hurries across the bridge, temporarily forgetting about Vasor. He gives Susan and Sabetha warm furs to wear and the girls start to explain about the terrifying ice soldiers.

Altos is first to catch sight of Vasor as he unhooks the rope bridge on his side. He drops his end into the chasm, trapping them all on the far side. He tells them there is no way out; safe now from the "demons", he leaves them all to die. Ian is furious with himself for leaving Vasor there. Barbara makes a helpful suggestion - if they can find logs or planks, they may be able to make a crude bridge to get across. Ian is still angry, but goes along. They head into the tunnel to search for suitable material.

The tunnel is a short slope which leads to another low opening. However, the opening is topped by a huge chunk of ice, kept in place by a much smaller and more fragile piece. It is very dangerous indeed and Ian councils caution as they all go through. Susan wishes to go back to avoid facing the fearsome soldiers again, but Ian is convinced that the microkey must be hidden in here somewhere. It seems quite a leap, but he clings to that hope and urges them to press on.

They all emerge into the chamber with the soldiers. On closer inspection, it seems clear that the soldiers are long dead, frozen in place. Ian believes they are the basis for Vasor's "demon" legend. But Sabetha's eye is caught by something else: the microkey! It is frozen in the centre of the huge block of ice. Again they cannot believe their luck, but they waste little time wondering. Barbara notes the network of pipes which run all around the block. Susan finds their source, which passes right through the ice floor. Barbara finds a valve and impulsively turns it. As expected, heat begins to flow through. It must be linked to a volcanic spring like the ones in Iceland back home - geothermal energy. Susan and Altos report that it's already starting to melt the ice. Pleased, Ian suggests trying to repair the bridge while they wait.

However, they are unable to find any planks to go across. Instead, they settle on ice stalactites which they tie together in hopes they'll freeze together and become more solid. Some time later, Ian and Altos have placed three of the giant icicles across the chasm and go to look for more. The girls are in the chamber, monitoring the melting of the ice.

To their delight, the block is now almost completely melted and Sabetha goes to retrieve the key. The soldiers, too, have thawed out, slumping where they stand. Susan is admiring their fearsome swords and pikes when suddenly one of them comes to life, opening its eyes to stare at her. They were not dead but in suspended animation. Their task was to defend the key, and now that they are all awake again, they will continue to do so.

Susan screams as the soldier slowly raises his sword. The other three also begin to stir. Ian comes running and takes in the situation. He urges them all to run for their lives. Sabetha stumbles as she goes and is nearly decapitated, but Ian helps her out just in time. Slowly but implacably, the soldiers follow their quarry.

Susan and Sabetha wait near the chasm, unable to see what is happening inside. Knowing they must do something to escape, Susan takes it upon herself to try and cross the precarious ice bridge, carrying the loose end of the rope bridge with her.

As the soldiers warm up, they become more agile and Ian must use a spar of ice to defend himself and hold them back.

Barbara arrives and is horrified to see Susan trying to cross the crumbling icicles. Altos knows it hasn't had time to freeze solid yet. One of the three planks breaks loose and crashes down the chasm. Susan flattens herself against the remaining ice in fright but then forces herself to continue.

Ian races out of the cavern and through the narrow tunnel entrance. He triggers the keystone piece of ice and blocks the tunnel. He must still engage the relentless soldiers through the gap. He knows the blockage won't hold them for long.

Susan manages to make it across the chasm just as the entire ice bridge breaks apart. She is safely across, much to the relief of her friends. She quickly sets about retying the rope bridge.

In the tunnel, the inhuman ice soldiers free the blockage and continue their pursuit.

Ian is the last one across the rope bridge and he sends the others on as he once again disconnects the bridge. One of the ice soldiers bursts from the tunnel at a run and is unable to halt his forward motion on the icy ground. He falls over the edge to his death far below. The other three stop at the edge, unable to cross for the moment.

Back at Vasor's cabin, the trapper surveys his prizes - four travel dials and three microkeys. His conscience doesn't bother him one bit. However, he is stunned when Ian and the others burst inside - very much alive - to reclaim their property. All of them put on their travel dials and prepare to leave. They will soon be reunited with the Doctor.

Vasor looks outside the opened door and is horrified to see the ice soldiers - his "demons" - descending on his cabin. Somehow they crossed the chasm. He bars the door, crying out in terror. Ian is unsympathetic, ready to leave him to face the soldiers on his own. Desperate, Vasor grabs Susan in an iron grip. If he dies, they die too. Ian closes in on him, backing him against the door. Suddenly, Vasor's grip loosens and he grimaces in pain. No one had noticed their pounding on the door. Vasor drops dead and they all now see a sword protruding through the door. The soldiers are breaking in.

Ian exhorts his friends to turn their travel dials. They do so and disappear just before the soldiers burst inside. Their quarry is gone.

Ian reappears alone in a corridor in the city of Millennius. It is a modern metropolis. At the end of the corridor is what appears to be a vault door, ajar. All is quiet and still. Peering inside, he sees a black-clad man lying face-down on the floor. He checks the body and finds that the man is dead, bludgeoned. He supposed that the murder weapon is the ornate mace lying next to him. His gaze travels to upward and he sees - inside one of the several display cases - the 4th and final microkey. Forgetting about the body, Ian tries to open the case. Suddenly, he is struck on the head from behind and he falls unconscious next to the dead man.

Ian awakes with a king-sized headache, unaware of what has happened to him. He struggles to his feet and goes to the display case. As he heard, the microkey is gone. Suddenly an authoritative voice addresses him from behind, asking him where he hid the key. Ian whirls around to find a serious-looking man in a black military-style uniform sitting in a chair. He has obviously been waiting patiently for Ian to regain consciousness.

The man identifies himself as Tarron, an interrogator in the city's guardian division. He is single-minded and wastes no time in putting questions to Ian, despite his obvious pain. Ian explains that he entered the vault through the door, which was open, saw the body, and then was knocked out. Tarron finds this hard to believe. No one can enter this maximum-security area without a background check and the assistance of the duty guard. Ian reiterates his story, but Tarron twists his words, noting that the interview is being recorded.

Tarron takes a step back and gets Ian's name and occupation for the record. Ian also confirms he is a visitor to Millennius. Tarron asks if Ian knows the purpose of the missing microkey, a point he deliberately omitted from his story. Ian says he saw the key before he was hit and that's all. Clearly Tarron does not believe him. It was not found on his body or the body of the dead guard. He wants to know where Ian has hidden it, but Ian sticks to his story.

Believing that Ian is withholding evidence, Tarron orders the sealed door to be opened. He will now take Ian to the guardian HQ to formally charge him...with murder! Ian is shocked, never suspecting Tarron would leap to this conclusion. The situation is quickly getting away from him. Ian reiterates his story once again, noting the lump on the back of his head as proof. But for Tarron, the circumstantial evidence is enough. It seems likely to him that Ian also had an accomplice. Perhaps if Ian named him, things would go easier on him. But Ian is well-versed in the rule of law and reminds Tarron that the burden of proof is on the guardian. Evidence, witnesses, everything. Ian knows he is innocent until proven guilty. But he gets a nasty shock when Tarron coolly explains that is not how the law works here in Millennius. The evidence is enough to make him guilty, and the burden is on Ian to prove his innocence beyond a shadow of a doubt. Otherwise, he will face execution for the crime. Tarron suggests he find an advocate before the tribunal begins. Ian knows just the person - the Doctor - if he can be found in time.

Several days later, at the guardian's HQ, Barbara learns that she and her friends will be allowed to attend Ian's impending hearing, but they must not create any disruption or else they will be sent away to toil in the desert glass factories. They are also allowed to see Ian prior to the proceeding, but she is warned not to give him any items or packages. Barbara returns to sit next to Altos as Susan and Sabetha arrive. They have been trying for days to find the Doctor, but have had little luck. No one has seen him for 2 days. Susan wants to try and make a dash for it when Ian arrives, but Barbara thinks that unwise.

Ian is brought forward by a guard. He seems to be holding up and says he's being treated well. He is dismayed to learn that they haven't found the Doctor. He needs help badly, saying the laws here are a mockery of justice. The Doctor agrees...as he suddenly enters the room just in the nick of time!

Susan, Barbara, Altos, and Sabetha surround him, glad to have him back at last. The Doctor, too, is glad, but he knows they have serious business to attend to. It is clear that he has already heard of the predicament Ian faces. He wants to get all the facts from Ian, but it must be quick. The tribunal convenes in just a few minutes!

In the trial room, three ornately-dressed judges take their seats on a high bench. One of them speaks for all. In the gallery, guardians, citizens, and Ian's friends are seated to watch the proceedings. The judge announces that Tarron's report has been accepted by the court. From it, Ian is guilty as charged and has been sentenced to death. Unless he can prove his innocence to them, Ian will be executed 3 days from the end of this hearing.

The Doctor, as representative for the defense, is asked to stand. The law of Millennius allows any person to speak in representation of another, but the judges are concerned that this visitor is unfamiliar with their local laws to mount an adequate defense. The Doctor believes he has studied enough, but thanks the court for its indulgence. The prosecutor, Eyesen, does not speak as yet. His point of view is represented by the charges contained in Tarron's report. It is now the Doctor's turn to respond to the report. The Doctor says he has not had enough time to interview witnesses and assemble evidence and asks for a continuance. Prosecutor Eyesen objects, but the judge is inclined to grant the request. He notes that murder in Millennius is an unusual occurrence and every effort must be made to follow the law. The other two judges are in accord and the Doctor is given 2 days. The hearing ends and the judges depart.

Eyesen congratulates the Doctor on the small victory, but vows that it will not alter the trial's outcome. The Doctor is still confident, as is Ian. But the Doctor is troubled by something else - the man Ian is accused of murdering was Eprim, Altos' friend and another of Arbitan's searchers. The Doctor had already made contact with him and set a time table for obtaining the key. For some reason, Eprim went ahead of schedule and without the Doctor. And for it he was murdered. The Doctor surmises that Eprim must have revealed his plans to someone else and that person must be the true murderer. The Doctor has 2 days in which to prove it. He sends Altos and Sabetha to the library to research the rare murder cases in Millennius' history, trying to find precedents that might help Ian. They are to meet back here in 3 hours. Barbara and Susan will aid the Doctor as his detectives. For Ian, the hardest task is waiting. And he must trust the Doctor.

Altos and Sabetha hit the books immediately, poring over dozens of dusty volumes.

The Doctor, Susan, and Barbara have been granted access to the crime scene and there they interview Tarron. He repeats what he saw when arriving at the scene. He has rationalised Ian's bump on the head as either a fall or an early blow from Eprim before he died. The only part he can't understand is the absence of the microkey. It is not in the room and it was not on Ian, Eprim or anyone who went into the room. The Doctor assures Tarron the key must have been taken from the room somehow; it hasn't magically vanished.

Tarron relates to the Doctor what happened after the alarm bell went off. The relief guard was first on the scene, followed almost immediately by the controller and another guardian. He himself arrived shortly afterward. It seems hard to believe that they key could have been hidden in so short a time, but the Doctor is actually pleased with these facts. He believes that when they find the key's hiding place, they will also find the real murderer. Tarron is so secure in his belief that the real murderer is already in custody he actually allows the Doctor and his friends time alone in the vault. Nothing they can do will alter the outcome of Ian's trial.

As soon as Tarron is gone, the Doctor goes deep into his thoughts. Barbara snaps him out of it by asking questions. The Doctor reveals, quite casually, that he now not only knows where the missing key is hidden, but also is fairly certain who the killer is. Tarron, settled on Ian as the only possible murderer, would never have figured out what happened. The Doctor proceeds to reenact Ian's entry into the room to illustrate his theory. Susan takes the place of Eprim's body and Barbara takes on Ian's role. The Doctor directs the action from the door, where he believes the real murderer must have been hiding. He takes on the role of the murderer and knocks Barbara down. He mimes the lifting of the key. Knowing he had only a few seconds, the killer concealed the key (the Doctor does not say where) and then must have run to the door and closed it. With officials and guardians already on the way, the only logical thing for the killer - the relief guard himself - is to pretend to be first on the scene, reopening the door as the others arrive. That is why Tarron did not believe Ian's story that the door was ajar.

It all makes perfect sense to the Doctor and his friends. The relief guard was in league with Eprim all the time, the only way in which Eprim could gain access to the key. For some reason, the guard killed his partner but was interrupted by Ian's arrival. He was forced to knock Ian out, but then used his timely arrival to frame Ian for the crime. Susan and Barbara are ready to tell Tarron, but the Doctor stops them. This is only an unproven theory right now. Even revealing the location of the stolen key would only serve to implicate Ian at this point. He then tells the women that he has an errand for them, one which they will find very very interesting.

In an apartment elsewhere in the city, the wife of relief guard Aydan, a hard and serious young woman called Kala, answers a buzzer at her door. It is Susan and Barbara. Aydan is not at home, but she plays the charming hostess and invites them in. They tell her they are here to interview Aydan about the case, but Kala thinks this a waste of time. The guardians have already questioned him thoroughly over the death of Eprim, a good friend. She assures them he held nothing back.

Barbara agrees with her, but then turns to the questioning to Kala. Why, Barbara asks, would someone want to steal the microkey. Kala says it is one of only 5 in the entire universe, entrusted to the city long ago by Arbitan. It is rare and valuable. Barbara speculates that the motive was ransom, blackmailing the city to retrieve it. Kala dismisses this speculation, thinking they should know the real motive as it was their friend Ian who stole it.

Aydan returns, grumbling about the grilling he received from the guardians. He stops short and becomes even angrier when he catches sight of Susan and Barbara. He demands that they leave immediately. Kala tries to calm him, but Susan needles him further about his upcoming testimony and then blurts out that they know where the key is hidden. Angered, Aydan starts to give a response that sounds very incriminating indeed, but he catches himself. Instead, he says that Ian must have told them where he hid it; that's how they know. Barbara and Susan taunt him further and the volatile man raises his fist to strike Susan. Kala stops him and urges the women to leave.

Susan and Barbara exit but stay just outside the door. A row develops between a clearly-spooked Aydan and his much calmer wife. When she rebukes him for losing his temper, he slaps her into silence. Through a rather large keyhole, Barbara can see one of them dialing the phone a moment later. But she cannot watch further as footsteps approach. She and Susan are forced to leave.

In the justice building, Prosecutor Eyesen receives a call. He takes it on a personal line; it is unclear to whom he is speaking. He receives a report about Susan and Barbara's visit to Aydan's home and is afraid that "their young friend" might not be able to go through with the plan. He outlines a course of action that may become necessary in that event. Clearly Eyesen is implicated in the crime Ian stumbled into.

When the trial recommences, Eyesen concludes with presentation of the mace - a primitive relic from earlier times - as the weapon which killed Eprim. A psychometric examination revealed that the last person to hold the mace was Ian. He rests his case, certain he has substantiated the conviction with his evidence. Then it is the Doctor's turn. With all the information he has gathered, the Doctor has decided on shock tactics to mount his case. He announces that the true murder is in the courtroom, sending a ripple through the gallery, but then adds that he is not under arrest. He promises to reveal the true murderer's identity in due course, but calls a witness first.

Oddly, this witness is Sabetha, who calmly takes the stand. All are confused as she seemingly has nothing to do with the case. The Doctor produces a drawing of the missing microkey and asks Sabetha if she knows where it is. She says it is in her possession and she produces it to the astonishment of everyone present. The Doctor questions her further and she says it was given to her by the murderer. When asked to identify him, she points to Aydan, sitting in the front row of the gallery.

Aydan stands, looking nervous and very guilty indeed. He cannot believe that Sabetha has the stolen key, but stops short of saying anything incriminating. He bolts for the door but is caught by Susan, Barbara, and others in the gallery as pandemonium breaks loose. Unable to run, he shouts that he will tell them everything. He did not act alone in the crime, but says that "they" forced him to do it. However, before he can speak further, he is hit by an energy beam and falls to the floor, dead. There is a moment of stunned silence before Kala begins weeping and falls upon the body of her dead husband.

Later, when order has been restored, the Doctor admits privately to Eyesen and the judges that he perpetrated a ruse to get Aydan to confess. Sabetha merely showed one of the other keys their group has collected in their travels and not the actual one that was stolen in Millennius. He explains briefly about their quest. The fact remains that the final key is still missing as far as the officials are concerned (the Doctor maintaining privately that he knows where it is).

Despite Aydan's confession, Ian's ordeal is far from over. Eyesen still thinks he is guilty, as do the judges. There are several matters they still wish to clear up.

Meanwhile, Ian and Barbara speak with Tarron regarding his investigation into Aydan's murder. The weapon has been recovered and psychometric tests should be able to determine who fired it. Obviously, whoever killed him was about to be implicated in the theft of the key and the murder of Eprim. Aydan's death silenced him at a critical moment.

Soon the tribunal reconvenes and everyone takes their places. Everyone, that is, but Susan. Barbara tells Altos she has gone to fetch a written statement from Ian which the Doctor inadvertently left behind. The judge announces that Aydan's involvement in the theft and murder is obvious, as is the existence of accomplices. The evidence presented thus far makes Ian a prime suspect as an accomplice and his guilt has become even more obvious. Eyesen summarises the case against Ian with these new facts, even implicating Ian's friends in the plot and in Aydan's murder. For these reasons, he says, Ian is still guilty and the sentence should be carried out. The judges all concur, but Barbara, Sabetha, and Altos do not hear this pronouncement. They have been called out of the chamber by a guardian.

Unaware that his companions have left, the Doctor rises for his closing statement. However, knowing that things look bad, he asks instead for another continuance to gather new evidence. The judge denies the request and closes the case. Ian will be executed for murder under the provisions of the law. The execution is scheduled not for 3 days hence as originally planned, but tomorrow!

Sabetha thinks they should tell Tarron and the other authorities about Susan's kidnapping, but Barbara resists this idea. She does not want the Doctor to find out. He must keep his mind clear and focused solely on defending Ian. If he knows his granddaughter is in danger, it will distract him. Besides which, how do they know that Tarron is not involved. They can trust only themselves now and they must find Susan on their own. Sabetha and Altos agree, warning that they are dealing with callous murderers and must work quickly. With any luck, finding Susan will provide final proof of Ian's innocence.

Ian now must wait in prison, with only a guardian for company. On the wall, a bleeping clock counts down to the next zenith, when his execution will take place. He is not allowed to see his friends again. He is utterly alone.

Barbara has been going over every aspect of this crime and all that has happened to them since they arrived here. She comes up with Aydan as the link - the only one who knew all the conspirators. There is just a chance his wife would know his associates, even though she didn't know what was being planned. Hopefully, she will tell them in hopes of solving her husband's murder. Clinging to this slim hope, Barbara and the others hurry to Kala's apartment.

Kala reluctantly admits them, looking worn and frazzled. Barbara tells her what has happened, linking Susan's kidnapping to the people who killed her husband. Kala does not, unfortunately, know anyone with whom her husband associated; he was very secretive. Barbara's continued questioning causes Kala to break down in tears for her husband, dead and disgraced. She will have to live with both his loss and his crime for the rest of her life. She begs them to leave her alone. Barbara apologises, telling Kala she was their only hope. Kala collects herself long enough to empathise with their plight; they must all be sick with worry since they spoke to Susan, but Kala can do nothing to help. She begs them again to go, dissolving into tears.

Barbara, Altos, and Sabetha leave sullenly, unable to comfort the weeping widow or to deal with the dashing of their hopes. Kala continues to weep, but only until the door closes behind them. Then the false waterworks cease and a wicked smile lights up her face. She walks to a nearby door and slides it open. Behind it, Susan sits gagged and tied to a chair. Kala was the mastermind behind the entire plot from beginning to end.

The woman callously tells Susan that her friends stopped by but that they have gone away none the wiser. Her friends are like everyone else here - stupid. Kala is very confident. Her phone rings and she answers it in her weeping widow voice until she hears who it is - Eyesen, her co-conspirator. Over the speaker phone, Eyesen informs her of the verdict and Ian's impending execution. She is relieved to hear that the Doctor did not reveal the location of the stolen key. Eyesen is convinced that he never knew it at all. It was just another bluff. He tells Kala he will collect the key later tonight and then pick her up. Their plan will succeed. He tells her that Susan is of no further use to them and should be killed immediately. Susan cannot help but overhear this.

Barbara, Altos and Sabetha walk sadly from Kala's apartment. They have no more leads and they must tell the Doctor. Barbara blames herself for not getting help when she first heard from Susan, but Sabetha doesn't blame her as Susan sounded so frightened on the phone. They had to act immediately. However, this line of conversation suddenly resonates in Barbara's memory. Kala! She made mention of the phone call from Susan just now. However, they have deliberately avoided telling anyone and Kala couldn't have known of the call...unless she was with Susan at the time. Hopeful - and still worried - they all race back to Kala's apartment.

Kala removes a gun from a drawer and advances on Susan. She takes but a second to mock Susan and the courtroom tactics of her grandfather before moving forward to finish the job. But Altos, Barbara, and Sabetha have entered and crept up behind her. Altos disarms her easily and he and Sabetha subdue her. Barbara releases a grateful Susan.

Outside the courtroom, the Doctor sits dejectedly on a bench. There is no more he can do to save his friend. Eyesen leaves the courtroom and notices him. The prosecutor, with false sincerely, congratulates the Doctor on his work, and the Doctor returns the complement. There is a note of sadness in Eyesen's voice, playing his part to the hilt as he wishes he and the Doctor could have met under happier circumstances. He bids the Doctor goodbye and collects his things to leave for the night. He watches disinterestedly as the court documents and exhibits - including the mace - are locked up in a cabinet for safekeeping. Then he is gone for the night.

Once the cabinet is locked, Tarron turns to the Doctor tells him it is time to leave. But the Doctor's mind is still searching for some way to reopen the case and save Ian. A phone call comes in for Tarron. It is Barbara, telling him that Kala killed her own husband and that she is now in their custody. Tarron and a guardian hurry off.

A short time later, Barbara and the others fill in the details for the Doctor. Barbara apologises for not telling him of Susan's disappearance, but to him all that matters is that Susan is safe and well now. Pity the same cannot be said of Ian. Altos is hopeful that Kala's confession will finally exonerate Ian of the crimes.

Tarron arrives, fresh from his interrogation of Kala. She has indeed confessed to everything and has named her accomplice - Ian! The execution will go ahead as scheduled. The Doctor and his friends protest most strongly and even Tarron must admit he thinks Kala is lying. However, without proof he can do nothing for Ian. Susan reminds them all of the man who called Kala on the phone and ordered Susan's death. He's the true co-conspirator, but she did not recognise the voice. She does tell them, though, of his plan to collect the key tonight. The Doctor is jubilant. He is certain the man does not yet know of Kala's arrest and will still come to collect the key tonight as planned. And since the Doctor does indeed know where the key is hidden, they can catch the villain red-handed and finally save Ian.

As the night wears on, Ian is totally alone. The timer continues its relentless countdown. He cannot sleep and will not tonight.

Later that night, the court building is in darkness. The Doctor hides in the shadows, watching as at last his prediction comes true. A man clad all in black - including a mask over his face - sneaks into the reception area and makes for the evidence cabinet. He has a key and opens the cabinet, avoiding everything inside but the mace. Suddenly Tarron and a guardian burst from hiding and capture the man. The Doctor turns on the light and reveals the mastermind of the entire plot - Prosecutor Eyesen. This is at last enough evidence to stop Ian's execution.

Once Ian is released and brought to rejoin his friends, the Doctor reveals the key hidden inside the head of the mace. The Doctor knew all along that it was the only thing in the room not searched. Tarron is astonished at the Doctor's brilliance. He reports that Eyesen and Kala owned up to the entire plot - they planned to steal the key and sell it. But Ian got in the way and became their fall guy. Tarron never doubted Ian's guilt for a moment. The Doctor admonishes Tarron to read up on Pyrrho's scepticism, calling it a great asset to his business. As for the Doctor, he learned of it first hand, when he met Pyrrho!

The four travellers are overjoyed to be reunited and soon they will see Altos and Sabetha again too. The Doctor has sent them ahead to let Arbitan know they've been successful. He was probably very pleased to see his daughter again as well. Tarron presents them with the final key and a court authorisation to take it back to Arbitan. When Tarron also produces Ian's travel dial and the group is ready to go.

Tarron and the other guardian are a bit concerned as to how they will return to the island, seeming dubious at the idea of travel dials. But Ian, Barbara, and Susan prove their effectiveness by vanishing. The Doctor says a brief goodbye to Tarron and then leaves himself. Tarron is certain he cannot enter disappearing people into his official report and plans to simply say they left...to return the keys to their keeper, Arbitan.

Back on the island, the Voord have taken control of Arbitan's fortress and the conscience machine. Their leader, a raving fanatic of a man called Yartek, has even appropriated Arbitan's robes. He and his fellow wet-suited creatures have been waiting for the travellers to return, building up their forces here for the moment when they can control the entire planet. The Voord have captured an unsuspecting Altos and they hold him in the control room. He learns from Yartek that Arbitan is dead. Yartek has 4 of the keys and awaits the last. Altos' primary concern is Sabetha. Knowing she would never willingly part with the keys, he is afraid that she has been harmed.

Sabetha is brought in, alive and well, but quite defiant. In order to save Altos, she creates the story that he is a lowly servant who knows nothing of the keys and is of little value. However, Yartek sees through this and orders the "servant" killed. Sabetha is forced to intervene to stop it. Yartek grows angry at this petty deception and demands to know where the final key is. Sabetha refuses to answer, so Yartek turns to Altos. He has travelled with Sabetha and - obviously - is in love with her. To safeguard her life, he will speak. When Sabetha's life is threatened, Altos has no choice but to tell Yartek that the Doctor has the final key. Sabetha bursts into tears.

In the corridors of the pyramid, the Doctor meets back up with his companions at last, nearly frightening each other to death in the process. The Doctor has just come from checking on the TARDIS - his first priority. Arbitan kept his promise and removed the force field. The ship appears fine. Now they must go and find Arbitan. Susan and the Doctor head off, but Ian and Barbara lag behind. Both find it odd that Altos and Sabetha are not here to greet them. They are on their guard.

The arrival of the four travellers has been noted by Yartek and the Voord. With Altos and Sabetha safely hidden away, Yartek awaits the final key. He takes a moment to place the four keys into the conscience machine. Its power begins to build immediately. One final key is all that's left.

However, the time travellers become aware of the Voord presence when one of the creatures, ignoring his leader's orders to stay out of sight, attacks Ian. But they are ready for him and the Doctor knocks him out cold. It is as they'd feared. The Voord are here in force. The Doctor is ready to return to the ship, but the others wish to stay and find Altos and Sabetha. They may be in trouble, to say nothing of Arbitan. The group splits up, Ian and Susan going in search of Arbitan, the Doctor and Barbara in search of Altos and Sabetha. Ian takes the microkey from the Doctor in case they find Arbitan, but the Doctor warns him to give it only into the right hands. Then they are off, Barbara taking the Voord's knife just in case.

In their cell, Altos and Sabetha are firmly tied up. Altos berates himself for betraying the Doctor...and her. He struggles to free his hands, if not to escape (for the bars look solid indeed), then to at least defend them from the Voord. Sabetha is just glad that he is with her and she is not alone.

In the conscience room, Yartek has drawn his hood over his rubbery head, concealing his identity. He knows Ian and Susan are coming and pretends to be Arbitan when they arrive. They are taken in for the moment, even though "Arbitan" behaves strangely, demanding the key without so much as a greeting. When Susan mentions his brusqueness, Yartek reels himself in and lowers his voice, asking forgiveness for his understandable impatience. He tells them that Sabetha is safe and well. However, when Ian approaches, "Arbitan" cringes away. He says that he is suffering from a terrible disease caused by the energy from the conscience machine. No one must come near him and only Sabetha knows the cure.

Ian inquires about Altos and "Arbitan" becomes quite parental. He wishes to know if the young man in love with his daughter is a good and true person. "Arbitan" wishes to know Ian's impressions of him. Ian gives a glowing report, but says that "Arbitan" must decide for himself once he gets to know the young man.

The niceties over, "Arbitan" asks again about the final key. Ian produces it and despite Susan's hushed objections, gives it to him. "Arbitan" asks for the Doctor to be brought and he will summon Sabetha. They must all enjoy their "triumph" together. Ian goes off with Susan to fetch the Doctor. Yartek cannot believe the gullibility of these humans. He removes his hood and addresses a subordinate. He does not want Ian and the others captured, preferring instead to let the machine enslave them. Instead he sends the creature to fetch Sabetha and Altos. He wants them here before he inserts the final key and completes his conquest of Marinus.

Susan is convinced that they were not talking to Arbitan at all. Ian feels the same, sure that the real Arbitan would not consider Altos a stranger. After all, Arbitan supposedly sent Altos after the keys himself. Susan then wonders why Ian gave this unknown impostor the final key. Before he can answer, Barbara finds them both, taking them to meet up with the Doctor.

The Doctor has found and freed Altos and Sabetha, learning all about Yartek and his plans. Barbara fills Ian and Susan in as they go and soon all are reunited in the cell. Susan announces with dread that Ian gave the final key to Yartek and the Doctor is aghast. However, Ian tells them with a sly smile that he gave Yartek the fake key they found. He still has the real one with him. The Doctor is pleased, knowing they've won, but Altos and Sabetha are gravely concerned. The fake key, once inserted, will activate the conscience machine, but the energy that surges through it will cause the machine to explode, taking the building with it! They all rush to escape.

However, they have trouble finding the concealed exits and must search desperately along the walls. So intent are they that they do not notice the Voord who observes them.

The Voord reports this to Yartek. The fact that they know Arbitan is dead means nothing. They cannot escape the power of the conscience machine. His moment of triumph at hand, Yartek inserts the final key. The machine explodes almost instantly, killing Yartek.