On an unnamed planet in an unknown time, a small race of people called Gonds celebrate a most important ceremony: the election of companions. Selris, leader of the Gonds, receives a tablet from inside a machine and from it reads the names of the two young people selected. One male - Abu - and one female - Vana. They alone in their class have been chosen as the best and brightest. Their friends are suitably proud of them and offer their congratulations. All but one.

An irate young man, called Thara, pushes his way to the front of the crowd and expresses his displeasure at the selection of Vana, his beloved. He goes to speak with her urgently as Selris tries to continue the ceremony. Abu and Vana are to receive the highest honour of the Gond people - they are to become "companions of the Krotons". They are to step forward for the investiture of their robes.

Abu duly steps up and is fitted with a silver cloak and a metal necklace by Selris' assistant Eelek. However, Vana is being held back by Thara, who tries to convince her to refuse the "honour". It is clear he sees no value in her selection and asks aloud why she should have to obey. Selris puts a hand on the hot-headed young man and stays him. Selris reminds Thara - his own son - that Vana must obey. They all must. It is the law of the Krotons.

All eyes turn as a door slides open on the side of the machine and Abu walks inside. The door closes behind him and the only sound in the Hall of Learning is that of the machine humming away.

In a grey, ashen landscape of hills, the TARDIS materialises. The Doctor emerges with a long stretch, as if waking from a nap. Jamie and Zoe have already been outside and don't like the looks of the place. The smell is particularly nasty - like bad eggs. The Doctor, however, wishes to explore, producing an umbrella against the glare of the twin suns high overhead. He denies the suggestion that the atmosphere might be poisonous as the instruments would have said so. Zoe worries about the pervading smell of sulphur but the Doctor's nose tells him it's all right - a bracing mix of ozone and sulphur. He leads off and the others hurry to catch up.

They scramble over the loose rock of several hills, the Doctor humming a little tune as they go. He stops to pick up a large piece of mica. Zoe doesn't share his curiosity about it and Jamie doesn't even know what it is. A few more steps bring them to the edge of another hill. The Doctor stops and smiles at what he sees below - a small settlement of tall, wedge-shaped buildings constructed out of rock. Despite the smell, this place isn't dead at all. The Doctor does admit the architecture is a bit odd for a planet with this high a gravity, but there could be a number of explanations for that. He wants to find out.

Elsewhere down the hill, Jamie spots something of interest and goes to investigate. He calls the others to him as he slides down the rocky slope. They all meet up in front of a hexagonal portal which seems to protrude from the rock itself. There is a door in it and a ramp leading off. The portal is a dull grey colour, looking like metal but it is covered with moss and lichen like a living thing. Jamie notices the sulphur smell is much stronger here. After a bit of investigation, the Doctor leads his companions away, a worried tone in his voice as he tells them this is some sort of machine.

However, they don't get far before they are stopped by the opening of the doorway. They hurry into hiding behind some rocks as a man emerges. He is young and tall, wearing a silver cloak and a metal ornament around his neck. Unbeknownst to them, this is Abu, the most recent companion of the Krotons. But he is much changed from earlier. His face is slack, his eyes blank, and he staggers as he tries to walk. He emerges from the machine and two long nozzles protrude form openings alongside. A thick gas pours from the nozzles, engulfing Abu. He screams in pain and falls from the ramp. The Doctor, Jamie, and Zoe can do nothing but watch in horror as the gas consumes and dissolves the man. When it is over, there is nothing left of his body but slag. Only the metal ornament survives.

They are all stunned and very frightened by this seemingly barbaric act. The Doctor urges his friends away from there, moving as fast as they can.

In the Learning Hall, Thara pleads his case to try and stop Vana from being sent into the machine. He will not accept that this is an "honour" conferred on her. To him, it is an ancient custom with little sense. He even goes so far as to question whether or not the Krotons really exist. With that, however, he has gone too far. Even Vana thinks so. Eelek has had enough and orders him to step aside. Thara refuses, drawing his axe, prepared to fight to keep Thara away from the Krotons.

At the back of the hall, the Doctor, Jamie, and Zoe enter, having found a way into the settlement from outside. They see the commotion and worry what they've stumbled into, but they can tell by the clothing that the man they saw killed was part of this group. The Doctor puts on a big smile and tries to convince the Gonds that they come in peace.

Clearly, the Gonds are not used to visitors because the reaction is less than hospitable. Pikes are wielded against them and the Doctor is unable to speak. They are immediately pegged as strangers and asked to give an account of themselves. Jamie becomes angry and the Doctor tries to calm everyone down. It is Zoe who finally silences them all, pointing toward the front of the room and the drama unfolding there.

Eelek holds his ground in the face of Thara's armed challenge and demands that Selris rein in his son. Thara refuses to budge, shielding Vana from them. He won't let her go in - no one ever comes back when they join the Krotons. He will never see his beloved again if she goes in. In the end, it is neither Selris nor Eelek who defeat Thara, it is all the guards with sharp pikes pointing at his head. He is outmatched and even Vana knows he will not defeat them. He drops his axe and lets Vana pass, his face grim.

Vana already has her cloak and Eelek puts the medallion on her, breathing a sigh of relief. He shudders to think what could have happened if they had left their benefactors the Krotons waiting any longer.

Zoe recognises the doorway to which Vana is being led - it is the same as the one they saw outside. And Jamie notes Vana is wearing a silver cloak just like the man they saw killed. Clearly she is destined to experience the same fate. It is as if they are sacrificing the girl for some reason, although the Doctor thinks them too sophisticated a race to do that. But whatever the reason, they do know that girl will die if they don't stop her.

The Doctor shouts out a warning but is grabbed by the guards. Jamie leaps to his protection but is also grabbed. Sizing up the enemy, Jamie picks out the leader and challenges him to a fight, one on one. The strategy works and the man, Axus, agrees to a fight. Jamie refuses any weapons so it is bare hands against a stone axe. Axus' advantage is short-lived as Jamie gains control of both his hands and they grapple. Superior strength prevails and Jamie disarms and Axus.

However, this delay has been too long and Vana is sent into the machine. With a final glance back at her beloved Thara, she enters. The door closes behind her and the ceremony is over.

Selris moves to the back of the room to investigate the commotion. He, too, is taken aback at the arrival of strangers. He also wants an accounting of them and the Doctor knows they will get no further without an answer. Zoe tells Selris that they are from another planet.

The Doctor insists on knowing what is happening and where Vana has gone. Seizing the initiative, Thara tells them bitterly that she has "gone to join the Krotons". The Krotons are behind the door, inside the machine. Selris and Thara resume their argument whilst the Doctor moves forward to examine the door. Eelek regards him suspiciously and won't answer his questions. He wants to know why the Doctor is so curious about their ceremony.

Finally the Doctor explains about the murder they witnessed. There is stunned silence, mainly disbelief from the Gonds. Jamie tries to make them understand the gravity of what they saw but only succeeds in sowing more distrust. He tells them they saw Abu "on the other side of the machine", outside. Suddenly all the Gonds back away from them, muttering about "the wasteland" and "contamination".

The Gonds tell them that the outside environment has been poisoned and that no one has set foot there. It is contaminated and the Doctor and his friends will die from it. After a moment's thought, the Doctor responds that while the outside may have been poisoned once, it is now quite safe. He is certain of it, but the Gonds are sceptical.

Jamie brings the Doctor's attention back to the urgent matter at hand - Vana. If she comes out of the machine on the other side, she'll be killed! The Doctor, Jamie, and Zoe race from the hall toward the so-called "wasteland", determined to save her. Amazingly, Thara joins them, much to Selris' displeasure. He is certain that they'll all die out there. Going outside is forbidden by the Krotons, the givers of all their law. For Thara, that's just one more reason to go!

At the outer doorway, there is no sign of Vana, which is good. She's not there yet, but the Doctor knows they haven't much time. Thara arrives and the Doctor presses him into service collecting rocks. Together they wedge the largest rocks into the sockets from which the gas nozzles will emerge. That is unfortunately all that can be done. It is not long before the machine opens and Vana emerges. Like Abu before, she seems dazed and unable to walk properly. Once she is out, the Doctor leaps up onto the ramp and grabs her, holding his unfurled umbrella behind him for protection.

The gas nozzles activate but are delayed by the obstructions in the sockets. Eventually they burst forth and begin spewing their dissolving gas, but Vana is already well clear and in the care of Thara. Unfortunately, the Doctor's umbrella catches the brunt of the spray and is destroyed, much to his chagrin. At least he himself is unharmed.

The same cannot be said of Vana. She stares upward blankly and cannot move or speak. She responds to nothing and the Doctor says she's nearly catatonic. He's not even sure there's anything he can do to help her. They must get her away from here and Thara offers his father's house, not far away. The entire group hurries off, Zoe sparing a concerned glance back at the deadly machine.

A little while later, Selris has welcomed them all in. The leader of the Gonds can hardly believe what he has learned about the Krotons. They have always been though of as benefactors. The apparent truth is very distressing. However, there's little room for doubt. All he needs to do is look at poor Vana to see what really happens to the "companions". Thara storms out, unable to bear Vana's suffering. Meanwhile, the Doctor works with hypnosis to try and reach Vana's damaged mind.

Selris explains that every so often, the 2 brightest students are invited to join the Krotons. They have always thought of it as a great honour. Could they really all have been murdered? With the wasteland forbidden and the spray dissolving its victims, there has been no evidence to prove anything. But it certainly looks that way now, despite the insensibility of it. Selris reveals that no one has ever seen the Krotons. They live in the machine and never come out, not for thousands of years. Not since "the beginning".

The Doctor completes his work, putting Vana into a deep sleep. He can only hope she will be all right in time. He joins the conversation and learns of the origins of the Krotons. Gond legend says that "silver men" came out of the sky and lived among them. The Gonds attacked them and the Krotons caused a poisonous rain to fall, killing hundreds and destroying the environment. The wasteland. Anyone who set foot there would die in terrible pain.

The Learning Hall is now dark and empty. The constant hum of the machines is louder now. Pervasive. The Custodian of the Learning Hall makes his evening rounds, checking all the machinery. From the darkness, a group of young men emerge and take him prisoner. The group is led by Thara.

The young men are united against the Krotons, their cause even stronger after today's events. They want to see the Krotons for themselves and they brandish their weapons at the frightened Custodian to gain his assistance. He assures them that the Krotons never come out of the machine - he is merely their servant and responds to messages and the occasional voice command. There is no way into their machine, except as companions. Thara reminds him bitterly what happens to them.

However, Thara accepts the Custodian's words. There is no way in to the Krotons. So, he and his friends resolve to fetch them out... by destroying the teaching machines. Horrified, the Custodian tries to cry out for help. Thara holds his men back from harming him, but he will be bound and gagged. They have no quarrel with him, only his masters. The Krotons.

Meanwhile, the Doctor learns about the teaching machines from Selris. Since the war, the Krotons have been at peace with the Gonds, educating them through the teaching machines. The Krotons provide all laws, science, and culture to the Gonds via the machines. The teachings also instill fear of disobedience to the Krotons. The Doctor calls it "self-perpetuating slavery". From this education, the two brightest students are selected to join the Krotons inside their machine. Now it appears that they have all been murdered by their "benefactors".

Selris sits heavily, scarcely able to take in the enormity of the truth. He agonises over what to tell his people. How can he tell them that all of their best and brightest have been murdered? He fears another deadly war with the Krotons. The Doctor cannot help him with that, nor can he explain it at present. But he does know that it's high time the slaughter was stopped.

They are joined by a man called Beta, Controller of Science. He seems agitated and cuts through Selris' introduction. He has learned of Thara's plot to wreck the Learning Hall and attack the Krotons. The war that Selris fears could already be starting, led by his own son. The Doctor suggests they hurry there, cutting through the wasteland to save time. Both Selris and Beta are nervous at the prospect, but the Doctor and his friends are living proof that the environment is safe. They all hurry off.

Thara and his friends have succeeded in destroying a number of the vital machines. Unbeknownst to them, they have provoked a reaction inside the machine. A small robotic controller comes to life as a warning signal flares. Action must be taken to curb the aggression against the machine.

Thara and the others turn to another teaching machine and start to smash it when a deep, commanding voice booms out from the Krotons' machine. It warns all Gonds to leave the Learning Hall at once. It is almost like the voice of God. They are awestruck. However, when the voice begins repeating itself, Thara realises it's just a recording and becomes even more angry. They set upon another teaching machine.

The Doctor and the others arrive and disarm the young men, halting the destruction. The Doctor does his best to explain to them how futile their actions are and how much danger they may face provoking the mighty Krotons.

Inside the machine, the activities outside are being monitored by the controller. It focuses in on the Doctor, on whom all Gond eyes are fixed. This is presumably the leader of the Gonds and the person responsible for the attack. It is the Doctor on which the robot centres its attention.

Thara cannot be convinced of the Doctor's words. He has little concept of science and the relative powers that the Krotons might possess. All he knows is that he wants to see a Kroton and demand an explanation of what happened to Vana, Abu and the others.

The group is startled by a movement in the machine's wall. Something is coming out all right, a giant, snake-like probe emerges, a light on its head acting like an eye. They don't know what it is, but they are sure it's dangerous.

The metal snake slides forward, scanning the nervous group, looking for something or someone. Inside, it fixes on the face of its perceived enemy: the Doctor. The probe moves forward, now with purpose. The group scatters but the snake-like thing stays pointing at the Doctor. It moves forward relentlessly and the Doctor retreats. He trips and falls, unable to escape.

The Doctor is helpless against the relentless snake-like probe. There is no place he can go that it can't reach. As it moves forward for the kill, the Doctor instinctively covers his face with his hands. Amazingly, the snake doesn't attack but hangs back, blinking and bleeping. The Doctor takes advantage of its indecision to back away and stand up. He keeps his face covered, having made a realisation. The weapon works on pattern recognition, and the pattern is his own face. If it can't see him, it can't attack!

Zoe realises this means the Krotons have some sore of surveillance over the Learning Hall. The Doctor hopes this means that there is some way to get in touch with them. However, as he explains his theory, he unconsciously lets his hands drop and uncovers his face. Once it can see him, the probe goes into attack mode to finish the job it started.

Unfortunately, one of Thara's young friends chooses this moment to step forward and attack the snake. He is caught fully by the dispersion blast meant for the Doctor and is vapourised on the spot. All that remains is his axe.

Inside the machine, the robot controller closes out its programme. Mission accomplished, or so it thinks.

The snake reels itself back inside and the Doctor realises it was programmed to kill only once. It must believe that it has succeeded in killing him. How stupid of it! Thara is horrified and angered anew over the senseless murder of a friend by the Krotons. He lashes out against their machine, itching to confront them. It is all Selris can do to hold him back, fearing the worst if the mighty Krotons are provoked.

The booming voice of the Krotons fills the Learning Hall. It demands that all Gonds leave at once. They have destroyed the Gonds' "leader" and all must now obey. Clearly this is no recording. The assembled Gonds move reflexively backward, even though Thara urges them to stay and fight. The voice repeats and Selris steps forward to command his people. They must leave. This time, no one argues.

Some time later, the Doctor examines the unconscious Vana using a solar-powered instrument borrowed from Beta. It is quite advanced for a people who know nothing of electricity. Both Zoe and the Doctor are intrigued by the gaps in the Gonds' knowledge, especially chemistry. Despite that, their medicine is quite good. Their knowledge is given them by the Krotons - the gaps are likely to be significant.

Selris arrives and gets a guardedly optimistic report on Vana. The Doctor wishes to enter the Learning Hall again if it's safe but he is particularly cagey about why. Jamie is not to come with them and the Doctor gives him the job of waiting for Vana to wake up. He is to give her some pills when she does. He is not happy about being left behind, nor is Selris. The Doctor is able to placate Jamie but not the leader of the Gonds. He insists on coming.

The Doctor and Selris set out but Jamie holds Zoe back. He's certain the Doctor is up to something and asks her to watch over him carefully. She leaves with a promise and Jamie sits down to wait with Vana.

A group of guards check out the Learning Hall ahead of their leader. Selris, the Doctor, and Zoe are then left inside. All is quiet now. The Doctor's attention is drawn to a hatch at the base of the stairs. Selris says it is an unused space known as the underhall. The Doctor wonders how far underneath the hall it goes and decides to investigate. Selris joins him in the dark, musty underhall. Zoe is left on her own in the Learning Hall.

Not one to let an opportunity go to waste, Zoe starts to look around. Her attention is drawn to one of the teaching machine, still intact, its screen glowing invitingly. She hesitates only a moment before sitting down, plopping on the headset and activating the machine. A series of very complicated mathematical equations appear and Zoe sets about solving them. It's all very easy for a genius of her calibre.

Inside the Krotons' machine, an alarm sounds and the robot controller is activated again. A meter on the console registers wildly in its upper reaches.

The Doctor and Selris emerge a bit later, dusting themselves down. To Selris, there was nothing there but the foundations of the machine. The Doctor saw something else entirely. Something very curious. However, before he can explain, he sees Zoe sitting at the machine. He hurries over to her.

Zoe is smiling blankly, still at the controls even though the test is over. The Doctor snatches off her headset and she stands, staring beatifically. The Doctor is horrified and tries to snap her out of it. She tells him how easy it all was and how pleased the Krotons were with her. The Doctor steps up his efforts. He reminds her how the machines have been used to enslave the Gonds and how they implant impressions on their minds. She must snap out of it! This seems to work and Zoe realises what she's done.

Selris draws their attention to the meter showing Zoe's score. The best Gond students register less than half that mark. He is amazed and Zoe beams smugly. The Doctor is not amused.

At Selris' house, Vana stirs and wakes with a start. Jamie tries to calm her but she is almost delirious, raving about the bores, burning her mind. Thara races in at this moment and helps her to lie back. But she is soon seeing it again, burning lights boring into her mind. The memory is terrible. Thara tells her she is home and she calms.

To his great relief, Vana recognises Thara. He tells her that nothing can hurt her here but she cannot stop reliving her trip into the machine. She becomes agitated again at the mention of Krotons and repeats her raving about bores burning her mind. She struggles to get free of them. Thara does all he can to hold her down and calm her. Jamie slips the two pills into some water and gives it to Vana to drink. Slowly she relaxes and drifts off to sleep.

Neither Jamie nor Thara can understand what she's said, but it must be significant. Jamie hurries off to fetch the Doctor.

The Doctor and Zoe examine the doorway to the Krotons' machine. It is crystalline but not brittle, very unusual. The Doctor tells Zoe what he saw in the underhall - the "foundations" are actually a root structure, indicating that this machine is really organic in nature. It is odd, but not impossible. Some simple crystal forms resemble simple viruses. Somehow, this machine is a living organism.

Suddenly a bell sounds and Selris hurries to the machine. There is a message from the Krotons. They have selected "ZoeGond" as a companion! They have mistaken Zoe for a Gond and she is summoned. To the Doctor's horror, Selris insists that she goes as disobedience will provoke the wrath of the Krotons. The Doctor chastises Zoe for meddling with the machine in the first place and storms off.

He is going to take the test too. He can't let her go in there alone. However, he is so flustered by the whole situation that he cannot even activate the machine without help. And even when he does, he gets the first equation wrong. Zoe is disappointed. He starts again. Selris points out that this is the most advanced machine but Zoe is sure the Doctor can handle it - he's almost as clever as she is!

However, he makes another mistake and the meter still reads zero. He must concentrate.

A short time later, the meter inside is registering wildly. He's done it.

The Doctor finishes, pleased with himself. He's scored more than Zoe, although she points out that he answered more questions than she did. The Doctor smiles benignly, basking under the subliminal praise of the Krotons. But he forces himself to snap out of it, knowing that it is a form of mind control. They must now wait to see if the Doctor is selected, although Zoe has already been summoned. He refuses to let her go in by herself.

As expected, the bell rings again and a message arrives. "DoctorGond" has also been selected as a companion. The door on the machine opens and it is time to go. They've started this and now they must see it through. Selris bids them a solemn farewell, convinced he will never see them again. How quickly he has accepted the fate of those "chosen". The doctor, though, is determined to survive. He and Zoe enter.

Jamie arrives in the hall just as the door closes behind them. What on Earth could be happening? He leaps forward but Selris stops him, telling him grimly what has happened. His friends are now companions of the Krotons.

Inside the machine, the Doctor and Zoe are let into a control area. The machine is obviously a spaceship of some sort, but there is no sign of a crew. The Doctor calls out but gets no answer. There are two chairs in one corner, illuminated. They are meant to sit down. With no other choice, they do so.

The Doctor produces a watch fob and gives one end of the metal chain to Zoe. She must hang onto it. He draws her attention upward where a force generator hangs over the chairs. They are going to be attacked and hopefully the chain will help absorb some of the power load. A moment later, the machine activates and they are pinned to their chairs. A bright light engulfs them. They are under attack.

Jamie is furious, unable to believe that Selris just let them go. The Krotons' orders mean nothing to him and he knows full well what happens to people who go into the machine. He races to the door, looking for a way in.

The attack intensifies, a burning light boring into their minds. The Doctor and Zoe struggle as best they can. The watch chain does seem to help, absorbing some of the energy. But it begins to smoke and glow. Soon it snaps, but by then the attack is over.

A section of wall fades away to reveal two takes of some slimy liquid, bubbling away.

Zoe and the Doctor wake up, much the worse for wear. However, they have avoided the severe mental damage that Vana suffered. Zoe believes that the Krotons have found a way to transfer mental power into pure energy. The Doctor agrees, saying that their energy was harnessed to trip a thermal switch. They both notice the tanks at the same time and get up to take a look.

The Doctor seems very worried as he now understands what is happening. Zoe can sense his fear and wants an explanation. The Doctor produces a bottle and takes a sample of the liquid in the tanks. It is a slurry - crystals in suspension. He calls it a type of primeval soup, just like the one that supported the beginnings of life on Earth.

Suddenly a figure begins to take shape in one of the tanks, a crystalline entity. The Doctor and Zoe back away in horror, racing for an open doorway. As they go, a pincer-like claw emerges from the tank. It is the hand of a Kroton, finally alive again.

The Doctor and Zoe race for the exit. The Doctor hopes to be able to analyse the slurry once they get out. Perhaps it will tell them more about the Krotons.

Like a newborn, the giant crystalline Kroton emerges from its tank and makes immediately for a nutrient source - a hose that Zoe noticed on the floor earlier. It connects to the creature's midsection, providing a lifeline to needed chemicals. It is now fully mobile - box-like with facets. Tall, two-armed with pincers. For much of the time that the Gonds have feared them, the Krotons have not even been alive. Now one is revived and a second Kroton forms in the other vat.

Jamie has had no luck attacking the doorway with his dirk. Selris tries to stop him but Jamie is determined to get inside. He spots a crowbar.

Two Krotons are now fully formed and mobile and are quick to get on task. They are looking for the "Gonds" who were last here in the control room. They are spotted on a scanner in the exit shaft, a maze of equipment and tubes hanging from the ceiling. They hurry through, unaware that they are being observed.

The Doctor and Zoe reach the exit door and find the photoelectric cell which controls it. However, it is not working. The Krotons must have deactivated it to keep them from escaping. The Doctor remembers the piece of mica he picked up when they first landed and uses it to trip the circuit, crystal working with crystal. It works and the door slides open. They can escape, but there is still the matter of the poison jets on the ramp.

The Krotons know this as well, and anticipate that these two "Gonds" will soon be dispursed.

But the Doctor realises that if they hurry out and jump off the sides of the ramp, they should be able to avoid the jets.

The Krotons watch on the scanner as the Doctor and Zoe escape. The jets never do activate as they have been disabled by one of the Krotons, the commander. It has realised that these are not Gonds after all and it wants them alive. They watch as the Doctor and Zoe race off into the wasteland. The Krotons are certain they can order their loyal Gonds to capture these others and bring them back.

It is clear that the Krotons have been asleep for a very long time. They have assumed that nothing has changed and that the Gonds are still in thrall to them. However, the presence of these other "biped animals" hints at possible changes. Scanning the Learning Hall, they find Selris, undoubtedly a Gond, and Jamie, not a Gond. The Krotons conclude that these two races are working in alliance, a possible danger to them. The Gonds are simple and harmless. Clearly the others possess powerful mental energy. Just what the Krotons need. They decide to capture the other non-Gond. Jamie.

The machine's door suddenly pops open and Jamie is ready to go inside. Selris tries to stop him but Jamie is determined.

He enters the darkened control area, crowbar held as a weapon. But a Kroton comes from nowhere and grabs him. He drops the crowbar as the vice-like pincer closes round his arm. Jamie is forced into one of the chairs and set up for a mental test. The Krotons brag of their superiority to these "animals" as the burning light bores into Jamie's mind.

Jamie writhes in pain as the light pours down on him. Abruptly the light goes out and he is released. He slumps forward, drained. The Kroton commander has spared him, realising Jamie still has "value" - he can give information on the two non-Gond "high-brains". The Doctor and Zoe.

On a scanner, the Krotons watch as the two make their way across the wasteland. Zoe is confused as they don't seem to be heading back to the Gond city. Instead, they are going back to the TARDIS. Zoe wonders if they are planning to leave, abandoning Jamie, but the Doctor seems sure Jamie is just fine for now. Reluctantly, she follows him.

Jamie comes to with a splitting headache, only to find the two giant, nightmarish Krotons standing over him and pelting him with questions. They want information about his friends. Jamie doesn't really answer, focusing instead on the fact that they've gotten away safely. The monitor screen closes in on their destination, the strange blue box in the wasteland. Under duress, Jamie tells them about the TARDIS.

The Krotons lock their dispersion unit onto the TARDIS but the commander does not give the order to fire. The Doctor and Zoe enter the ship and everyone, Jamie included, fear that they are going to leave the planet.

Back in the Gond city, much has happened in the Doctor's absence. In light of the knowledge of what really happens to "the companions" of the Krotons, a movement has sprung up among the people to start a war against the Krotons. Chief instigator is Selris' #2, Eelek. With his toady, Axus, they have gained enough support in the ruling council to feel confident that they represent the will of the people. Eelek, Axus, and several others come to visit Beta, controller of science, in his laboratory. They seek his input on a matter of war.

Beta is appalled at the suggestion, certain that any aggression against the Krotons will lead to the total annihilation of the Gonds. Eelek believes that they have advanced far beyond the "primitive" Gonds who previously attacked the Krotons and that this time they can win. Beta says that their "advances" are given to them by the Krotons - they won't have provided the Gonds with means to destroy them. As much as he wishes to be able to fight and become free, talking to the Doctor has shown him just how little Krotons have really taught them.

Eelek and Axus tell beta what has happened to the Doctor and his friends. All have gone into the Krotons' machine - they are now surely dead or mindless. This news rattles Beta. He wants to help but says he needs more time. He has learned from the Doctor some basic chemistry, a subject forbidden by the Krotons. In time, he is certain that he can develop a means to successfully fight their overlords. But Eelek has no more patience. He wants freedom for their people and he wants it now. Beta begs him to wait, fearing a massacre, and would like to hear Selris' thought on the matter. Coldly, Eelek orders him to forget Selris as leader and to obey him.

With the Doctor and Zoe inside the TARDIS, the Krotons dare not destroy it. They need the minds of the high-brains. The commander despatches his underling to go and fetch them back here. The creature take a large, barrel-shaped weapon in its pincers and heads for the door.

The commander asks Jamie a lot of technical questions about the TARDIS, which of course he is unable to answer. The commander determines that Jamie has no value. It then turns back to the controls to guide its comrade, who clearly needs pinpoint directions to traverse the wasteland.

Selris returns to his house where Thara still tends Vana. She is improving rapidly and should be all right by morning. Thara is horrified to learn that the Doctor and his friends went into the machine, but even more horrified that Selris did nothing to prevent them. He cannot believe they went in to spare the Gonds the wrath of the Krotons.

Thara has heard of Eelek's plans to take action against the Krotons and clearly approves. All his father can think about is talk. Vana awakes, still feverish and afraid. Thara calms her and she drifts back to sleep. But there is still an undercurrent of hostility between son and father, despite the silence.

Seeing the Kroton's attention focused on its task, Jamie tries to make a move toward the weapons rack. The commander stops him. Jamie inquires about his fate and learns that as he has no value, he will be dispersed.

Thara tells Selris that Eelek has taken over as council leader. He has won over the people by telling them - in detail - what the Krotons have been doing to them all these years, and by promising them a quick revenge. Selris is angered, knowing what this means for their people. Eelek is a hotheaded young man with too much ambition. He has turned the will of the people into an opportunity for himself and he must be stopped. Selris agrees with Thara that the Gonds want this war of vengeance, but Eelek's plan of marching on the machine is pure suicide. He himself has a better way that calls for stealth.

The Kroton commander continues giving detailed vector information to its comrade, guiding it to the TARDIS virtually step by step. Meanwhile, Jamie tries to glean some more information. Zoe and the Doctor are needed for their mental energy - to provide "transfer power" for the dynatrope, which is this machine. The Gonds have no high-brains and are insufficient for the task. That is why they have been killed. That is standard operating procedure for Krotons.

Realising that the Krotons have no compunction for killing "lesser" creatures, Jamie thinks again about escape.

Beta stands his ground against Eelek, realising that it is ambition, not patriotism, that has led him to this course. He thinks it must be very strange for the new "council leader" to actually feel popular for once! Beta knows that Eelek's plans to attack the Krotons will only lead to a repeat of the massacre they suffered 1000 years ago. Eelek angrily insists that they have better weapons now and can prevail. Beta reminds him of the weapons that created the wasteland. Compared to the Krotons, they are still in the stone age and the results will be the same.

Jamie continues to question the unusually chatty Kroton even as he sidles once again to the weapons rack. He learns that the Krotons "function permanently" unless they "exhaust". This is merely a reversion to basic molecules, but even that can be reanimated, as they have recently done. Thinking this somehow unfair, Jamie protests. In between vector readings to its comrade, the Kroton quotes procedure again: all waste matter - Jamie included - must be eliminated.

Jamie now has his hands on one of the large, barrel-shaped weapons.

Outside, the Kroton continues its trek to the TARDIS.

The Doctor and Zoe emerge from the TARDIS, having completed their analysis of the liquid from the Krotons' tank. It has revealed that the Krotons' life system is based on tellurium. The tanks were polarised centrifuges which they themselves activated with their mental energy. The Krotons have been waiting a thousand year in stasis waiting for enough mental power to reactivate them. The machine has operated all along, teaching and tapping the Gonds to keep itself running. It is a horrible sort of trap, but the Doctor does admire the ingenuity and the efficiency that has allowed the creatures to exist through time.

The Doctor carries with him a large carpet bag and he begins collecting rocks and other deposits from the ground. He is looking for sulphur. There are lots of it about, accounting for the rotten-egg smell Jamie noticed when they landed. He is mum about why he wants it, but does point out again that the Krotons' machine is made of tellurium.

Meanwhile, Zoe worries that they are being watched.

She is right. The Kroton is nearly in range of the TARDIS now. Jamie watches worriedly while his friends are sitting ducks. He urges them under his breath to get back in the TARDIS.

At the Doctor's request, Zoe rattles off all she knows about tellurium, but stops suddenly when she sees the Kroton right on top of them. The Doctor joins her, gaping in astonishment at the giant creature and its giant weapon. They are both to come with it back to the dynatrope. It is an order they cannot refuse.

Jamie at last releases the dispersion weapon from the rack and fires it at the Kroton commander. It seems to have little effect, only to anger the creature. It bears down on Jamie, intent on destroying him.

At the TARDIS, the Kroton suddenly begins swaying wildly and calling out for "direction point"." It has lost contact with its targets. Seizing the opportunity, the Doctor and Zoe race for the hills.

The Kroton commander returns to the control panel to aid its colleague. The scanner shows no sign of the Doctor and Zoe, but the TARDIS is still in vision. They will destroy it after all.

From above, Zoe and the Doctor watch. The Doctor surmises that the Krotons are nearly blind in bright light and require precise guidance from control to move at all. Under command, the creature points its weapon toward the TARDIS. It is engulfed in dispersion spray and when the smoke clears, the TARDIS is gone. Zoe is horrified.

Mission accomplished, the Kroton turns to go back to the dynatrope, reversing the vector readings it was given on the outbound trip. Jamie lies on the floor of the machine, pretending to be unconscious, awaiting a chance to escape.

As the Kroton moves away, the TARDIS rematerialises on a ledge higher up on the hill. Zoe is stunned but the Doctor is not surprised. It has a Hostile Action Displacement System (HADS) which moves the ship out of danger if attacked. Luckily, he remembered to set it this time, although there is clearly something wrong with its landing selector!

The Doctor is now ready to return to the Gond city. He doesn't reveal his plans but he does want to check up on Jamie.

Beta has had enough. Eelek's plan of frontal attack on the dynatrope is suicide. He will not go along and will not assist. His advice is to wait until real weapons can be developed. Selris enters the laboratory and echoes Beta's sentiment. The plan is suicide and he forbids it. Flexing his new muscle, Eelek orders his men to prepare slings and fireballs, their most sophisticated weapons. The men obey and Eelek goes with them, leaving Selris and Beta alone.

The two men commiserate, but Selris already has an alternative plan. Realising that he must retaliate for what the Krotons have done, he has decided to attack the machine from underneath - he plans to destabilise its very foundation in the underhall.

The second Kroton has returned to the dynatrope and they return to business. They have only a finite time before the mental energy stores are used up and they "exhaust". It is imperative that the Doctor and Zoe be recaptured before then. They set about determining exhaust time.

While they are occupied, Jamie gets up and creeps to the door. He hangs around just long enough to learn that the dynatrope and the Krotons will exhaust in 3 hours. With that, he escapes.

Vana has recovered almost fully from her ordeal. She is tired and a bit unsteady, but she is otherwise fine. Thara is pleased. They are packing their things to head for the hills. However, Thara is stunned to see the Doctor and Zoe arrive at the door. He and Vana are both overjoyed to see them still alive.

The Doctor is happy to see that Vana's recovered but he urges her to rest. He notices the bags and learns to his horror that the city's been evacuated. Thara tells him of Selris' plans to attack the supports holding up the Krotons' machine. The Doctor thinks this a very bad idea indeed. He must stop it, but first he must see Beta.

Selris and some men loyal to him are hard at work in the underhall, attaching large chains and pulleys to the supports that hold the dynatrope up. They are nearly finished.

At the laboratory, the Doctor gives Beta the sulphur he collected in the wasteland. Luckily, Beta has not chosen to evacuate, for the Doctor has a task for him. He gives Beta detailed instructions to create a certain chemical, completely forbidden by the Krotons. Beta is sure that he can do it though.

Zoe suddenly wonders where Jamie is and Thara tells her. With even more urgency, she and the Doctor hurry to the Learning Hall.

Buoyed by the return of the Doctor, Thara decides to stay in the city and fight. Vana too. Beta tries to talk them out of it, but they are determined. Vana fakes a fainting spell, saying she is unable to walk that far. She must stay.

On board the dynatrope, an alarm sounds. Something is causing the machine to move out of balance. The Krotons faceted heads rotate in alarm as they work to figure out what is happening.

Below, the chains are being pulled taut and the foundations supports are being pulled away. A large crack has appeared in the roof. Soon, the whole machine will come crashing through. Selris is triumphal, but the rest of the Gonds simply run for their lives as the entire building quakes.

The Doctor and Zoe push their way down past the retreating men, the Doctor yelling at Selris to stop this dangerous meddling. No one moves so the Doctor races down to unhook the chains himself. Zoe spots a large section of roof caving in and shouts a warning.

Thara and Vana make their way cautiously into the Learning Hall. There is massive damage up here and a strong undercurrent of continuing vibration. Thara heads for the underhall, asking Vana to stay up here for safety.

Zoe coughs and peers into the dust cloud obscuring the underhall as Thara joins her. He learns that the Doctor was trapped in the roof collapse and he venture into the rubble to try and find him.

Selris sees Vana and is horrified to discover that she and Thara stayed behind. As wounded Gonds from the digging party are led away, Selris goes down below to find his son and the Doctor.

Thara finds the Doctor beneath some rubble, battered but otherwise unharmed. The Doctor is very worried about the ongoing vibration from the dynatrope but before he can do anything, there is another quake and a small cave in. Both he and Thara are hit. The Doctor manages to avoid injury but Thara is knocked unconscious and a large piece of ceiling lays on his leg. The Doctor calls Zoe to help him move the stone.

Inside the dynatrope, Jamie heads for the exit shaft, the floor shifting beneath his feet with ever quake.

Meanwhile, the Krotons struggle to regain the balance of their craft. By feeding in emergency power, they achieve balance and can at last assess the situation. On the monitor, they see Gonds in the underhall and realise they have been attacked. They also see Zoe and the Doctor rescuing Thara. The two "high-brains" must be brought inside. One Kroton goes outside to fetch them.

In the Learning Hall, the Doctor examines Thara. His leg is broken and he is in great pain, but he will be fine. Zoe whips up a splint whilst the Doctor urges her to hurry. The vibrations from the dynatrope have stopped, indicating that the Krotons are now free to retaliate for the attack.

Selris laments that his plan has not worked, but the Doctor thinks it has. He points out a liquid leaking from part of the machine, a sure sign that it is no longer functioning properly. He only hopes they can finish the job and he asks Vana about the sulphuring acid that Beta is making. She gives him a bottle full, the first test batch, and he is pleased.

Zoe remembers Jamie, still inside the machine, and the Doctor becomes very worried. He can't be of much use to the Krotons - like the Gonds - and will likely suffer the same fate as the others unless they help him. Zoe and the Doctor head for the wasteland, leaving the acid bottle with Vana. She ponders this strange unknown "acid" substance.

Jamie has at last reached the exit door but it is firmly shut and will not yield to brute force. He's travelled with the Doctor long enough to know a door mechanism when he sees it, futuristic or not. He investigates the photo-electric cell. At his feet is the Doctor's piece of mica and he thinks that this must be the way he and Zoe got out. He tries to fit the mica into the cell.

Eelek, Axus, and their followers arrive in the Learning Hall to confront Selris. Seven men are missing, four are badly hurt, and two may die. And for what; the plan failed completely. Eelek is sure this fiasco will spell the end of Selris' support and he himself will be the sole ruler of the Gonds. Selris tries to point out the damage to the dynatrope but all Eelek sees is failure. His attack, with slings and fireballs, will produce victory.

When Eelek finds a dead Gond beneath some rubble, he loses control and brands Selris a traitor. His actions, in violations of Eelek's council leadership, have killed one of their people. He orders Selris arrested. For his part, Selris tries to make peace, knowing that division among them will just lead to their mutual undoing.

However, their argument is silenced when a Kroton emerges from the dynatrope. The Gonds are stunned to at last look upon the creatures who have enslaved them for 1000 years. None of them can even speak at first. The Kroton orders the Gonds to produce the two alien "high-brains" at once. When Eelek proves argumentative, the Kroton presents a show of force, using its dispersal weapon to disintegrate a Gond standing nearby. All are stunned.

Eelek learns from the Kroton that the "high-brains" are needed to operate the drive mechanism of the dynatrope. He realises that if they hand over the Doctor and Zoe, the Krotons will leave their world forever and no more Gonds will be harmed. Throwing compassion out the window, Eelek agrees to hand them over.

Jamie at last succeeds - more by luck than by skill - in opening the exit door. Unfortunately, it only opens a few inches. He'll have to crawl out.

Selris is of course outraged at Eelek's callousness. Will he really hand over their friends - two people who have risked their lives for the Gonds - in order to prevail? He most certainly will.

Jamie is pleased to see the Doctor and Zoe. Together, they extricate him from the dynatrope and jump off the ramp just as the dispersion unit activates. They are all safe and sound, but there is no time for explanations. The Doctor sends Jamie off to Beta's laboratory with a message to make the sulphuric acid in bulk. There is no time to lose. They'll meet again in the Learning Hall.

Some time later, Eelek learns that the Doctor and Zoe are returning and he and his men go to find them. Selris, Vana, and the injured Thara are kept under guard in the Learning Hall. Selris and Vana are incensed at this treatment of their friends, certain that Eelek is desperate to give the people a victory - any victory - to cement his hold on power. They are determined to warn the Doctor.

In the lab, Jamie has been pressed into service to help Beta make acid. The fumes are choking and neither man has any idea what they're doing. They're just following the Doctor's instructions to make it, hoping against hope that they don't blow themselves up or asphyxiate themselves.

Desperate, Selris distracts Axus whilst Vana tries to slip away to reach the Doctor. The ploy works for a moment, but Vana does not get far enough. Axus sees her and calls out to the guards at the top of the steps. She is captured and hustled back down, dropping the bottle of acid she took from the Doctor. Selris picks it up, wondering what good it will do them now.

Meanwhile, the Krotons estimate they have only 27 minutes of power left, then they and the dynatrope will exhaust.

The Doctor and Zoe make their way into the Learning Hall, seeing no one on the way. Eelek enters silently behind them. The Doctor's plan is to try and get back inside the dynatrope. Eelek offers to help. His guards capture them both.

The Krotons see the capture on their monitor and open the dynatrope to admit the "high-brains". Only 22 minutes left.

The Doctor sees what is happening and tries to resist. He wants to go in, but not without the bottle of acid he left with Vana earlier. To his horror, she doesn't have it any longer. The Krotons call out to order the "high-brains" in and they cannot resist any longer.

The Krotons watch on the monitor as Zoe and the Doctor are forced into the machine. They start their launch sequence. The commander resists the temptation to disperse the Gonds in the interest of conserving power and also because he knows there is another means of destroying them...

Vana calls out to Selris about the acid, telling him how vital it is to the Doctor. As the door to the dynatrope closes, Selris launches himself forward and throws himself inside.

The Doctor and Zoe arrive in the control room, drawing the attention of the Krotons. Selris races in but has only a moment to give the Doctor the bottle before the Krotons. Selris races in but has only a moment to give the Doctor the bottle before the Krotons open fire on the intruder. Selris dies with a scream, disintegrated like so many others. The Doctor and Zoe are horrified.

The Krotons get right to business, setting the intergalactic link in preparation for take off. Only 12 minutes left to exhaust time. Zoe thinks they should just let the machine run down, but the Doctor assures her that if that happens, the energy release will destroy half the planet, including the Gonds and themselves. They must find another way.

The Krotons reveal that the dynatrope's drive unit works on mental power and four "high-brains" are needed in relay to power it. Two of their crew were killed in a battle and the ship made an emergency landing on the nearest planet. Just following procedure. They set the dynatrope into perpetual stability, leeching on to the Gond civilisation to try and develop high-brains to reanimate the Krotons and launch the craft. Their minimal mental power was only enough to keep the machine in balance, not enough for transference. The "waste matter" - the mindless Gonds - were dispersed.

During this exchange, the Doctor secretly passes the bottle of acid to Zoe. His plan is not yet clear to her, but she takes the bottle. 9 minutes to exhaust time.

Beta and Jamie arrive in the Learning Hall with a great cauldron of sulphuric acid. Axus bars their way, mocking their efforts as a waste of time. The Doctor and Zoe are inside the machine, at the mercy of the Krotons. Jamie is furious at the cowardly Eelek for handing them over and he wants very much to make him pay, but Beta holds him back. Eelek and Axus start to leave the hall as the rest of their followers have done. When that machine takes off, the entire city will likely be destroyed. They're not planning to be buried alive.

Jamie decides to stay, determined to rescue his friends. Beta agrees to stay also, as does Vana. Eelek is content to let them all stay here and die together. With that, he is gone. Jamie, grim-faced, vows not to go down without a fight. He and Beta look for the best place to use their acid weapon.

The Krotons are impatient to be off and order the Doctor and Zoe to take up positions. The Doctor starts to stall, managing to impart his plan to Zoe as he talks nonsense. At his urging, Zoe secretly dumps the acid, bottle and all, into the tellurium tank from which the Krotons draw sustenance. When she is done, the Doctor is out of talk and they must take up positions. Zoe then stalls a bit, waiting for some sort of reaction from her contamination of the tank. Sadly there is nothing. They need more time. A bit of Abbott and Costello shtick buys them a bit more time but then they are forced to put on their headsets and submit to the dynatrope.

The extraction of mental energy begins and Zoe is transfixed. Suddenly the Krotons begin gagging and reacting to the poison. The result is quick as both of them stumble and fall. The Doctor removes Zoe's headset and hauls her to cover as the hand-held dispersion weapon carried by one of the Krotons goes off, firing wildly at the ceiling.

When the power is exhausted, they emerge. They see that the tellurium-based Krotons are dissolving from the acid, reverting to basic molecules with no hope of reanimation this time. To their astonishment, the whole dynatrope is doing the same thing, melting before their eyes. They hurry out before they're trapped.

Jamie, Beta, and several men loyal to Selris are pouring acid onto the dynatrope from outside. They have exploited the damage done earlier by Selris and targeted the leaking section. It is working quickly. The fumes from the sulphur and tellurium are choking, but Jamie manages to greet his friends warmly when they arrive.

Vana, supporting the injured Thara, watches as the acid does its work. The entire dynatrope is dissolving before their eyes.

Beta is praised for his quick thinking in using the acid as he did, but Zoe wants to know how the Doctor knew the machine and the Krotons would dissolve. He answers with a wink, explaining that while the dynatrope wasn't pure tellurium, neither was the acid pure sulphuric. He was always one step ahead. And he proves that once again by sneaking out of the Learning Hall while all Gond eyes are on the dynatrope. He's always hated goodbyes.

The dynatrope dissolves away and the Gonds are at last free. Thara will be proud to take over from his father as leader of the council and he'll gladly take care of the ambitious and cowardly Eelek. Beta is anxious to develop their own science, free of Kroton interference. He hopes to get some assistance from the Doctor as well, but they discover that he is already long gone. That suits Thara just fine. They'll have to find their own answers from now on.