The planet to which the TARDIS is being dragged down seems more like a moon than a proper planet. Its surface is barren and rocky, covered in crags and pitted with craters, and its atmosphere is thin and unmoving. If ever the term "alien" had a proper reference, this is the planet it was meant to refer to. It is here that the TARDIS finally materialises - amid a silent, web-covered plain.

The Doctor, Ian, and Barbara survey the dreary grey landscape on the scanner. Ian and Barbara note the lack of vegetation and movement, but the Doctor is more concerned about the force which dragged them here. Vicki joins them presently, the last to finish changing out of Roman garb. The Doctor rejects Ian's suggestion that there's something simply wrong with the TARDIS and decides to try counteracting the force with more power. Of concern to him is the nature of the force - intelligent and deliberate or mere natural phenomenon? As he sets to work, Vicki reacts in pain to a sound that only she can hear - it pierces her senses.

Outside, unseen by any of the time travellers, a group of creatures very like human-sized ants scurry about. They call to one another with a high-pitched chirping sound. It bounces off the rocks and amplifies itself in the thin atmosphere. This is the noise Vicki hears.

Ian and Barbara are concerned for the girl. She is obviously suffering and no one else can hear it. The Doctor thinks it is the type of extra-sonic sound that only children and animals can hear, but he's too absorbed in his work to pay much heed. Soon Vicki relaxes as the noise subsides, but she is still shaken by the experience. Barbara urges the Doctor to get them away from here. He is nearly ready to try, having boosted the ship's power output in preparation.

The Doctor activates the dematerialisation switch, but the magnetic power on this planet is too great. The ship starts to fade away and then is forced to return to solid form again. Outside, the giant ants have brought another creature forward - a sort of giant potato bug or grub creature which scuttles close to the ground and has a long pointed snout. The ants direct the other creature toward the TARDIS and it causes a strong webbing to go up around the police box shell. The ship is trapped on this nightmare world.

Main power in the ship shuts down of its own accord. Before the Doctor can investigate, all 4 travellers are flung across the room as the whole of the ship tilts and spins. Eventually it stops and they return to the console. Barbara notices movement on the scanner but it is gone before the others can look. The Doctor thinks it may just be the reflection of an errant cosmic ray. Barbara goes to help Vicki, who has been overcome again by the chirping noises. She takes the girl to her bunk to rest. The Doctor decides that they must go outside to find the source of the interference in order to properly counteract it. He and Ian will go, the women will stay behind. Ian tells Barbara, who has returned to the console room to find some aspirin for Vicki's headache. She is concerned, but Ian assures her that they will be fine.

The Doctor returns wearing an anorak over his regular clothes and carrying a spare one for Ian. The jackets have metal vents on the front which point upward toward the wearer's face. The Doctor calls them ADJ's - Atmospheric Density Jackets - which will compensate for the thin atmosphere outside and allow them to breathe. Ian is impressed with the technology. But there is one problem - how to open the doors without power to the console. Looking by turns guilty, shifty, and then smug, the Doctor manages to open them. He tells Ian that the dark-jewelled ring he always wears is more than just decorative, then moves to a piece of equipment along one wall of the room. He activates it and then waves his ring in front of a lighted eye on a stand. The doors glide silently open. Ian is stunned as he follows the Doctor outside. The old man cackles gleefully at his own cleverness as he goes. The doors close behind them.

Barbara brings Vicki two pills and some water. The girl is reluctant to take them and asks to see the bottle. She thinks they look "medieval" and likens them to leeches for Barbara's reference. Wherever they came from, she's never heard of "aspirin" in the 25th century. Barbara is interested in Vicki's apparent knowledge of medicine, but Vicki just assumed Barbara had such knowledge as well, being a teacher. Hearing about Coal Hill's "three R's", Vicki thinks it must have been a nursery school. Vicki herself took certificates in medicine, physics, and chemistry among others by age 10. They were taught by machines and had to study nearly an hour a week! Not sure whether she's being put on or not, Barbara puts an end to the discussion and urges Vicki to put aside her fears and take the pills. Otherwise she'll have to resort to voodoo and that would take too long to get the evil spirits out. Vicki laughs and takes the aspirin, grimacing at the awful taste.

Vicki notices Barbara toying with the bracelet on her arm. She knows it is new but has a hard time believing it was a gift from Nero. Ian and Barbara still have not managed to tell of their adventures in Rome. She promises to tell Vicki all about it when she wakes. Vicki promises to corroborate her story with Ian. If he returns.

Outside, the Doctor examines a rock formation near the TARDIS. It is composed entirely of mica, something he has not seen before. He wishes to examine it further and asks Ian for something sharp. Ian produces a gold pen, which suddenly leaps out of his hand and disappears before the Doctor can even touch it. The Doctor thinks it is some conjuring trick Ian is trying out, but Ian assures him it is not. His pen has disappeared. He raises his voice in indignation and a stream of strange distorted echoes returns his words to them. It is just one more element of mystery to this increasingly strange world. Ian also feels as though they're being watched, and while the Doctor doesn't share the feeling, he wouldn't blame any indigenous creatures for wanting to take a look at the newcomers. But why they would want to steal Ian's pen is unknown. Excited by the hint of mystery, the Doctor heads off happily to continue his investigations. Ian follows in a moment, after one glance backward to try and catch who or what may be following them.

In the TARDIS, Barbara returns the aspirin bottle to the first aid kit, marvelling at the strange things the Doctor has contained within. She looks toward the scanner to see the Doctor and Ian wandering off. Suddenly, she zones out and her left arm raises of its own accord, pointing toward the main doors. Barbara comes to herself shortly and gets control of her arm. Disconcerted by this, she returns to the living quarters and sits on her bunk. The movement wakes Vicki, who is concerned at the worried look creasing Barbara's face. Barbara chalks it up to the weirdness of this place, but she cannot hide it when her arm again moves by itself. She tells Vicki that her arm doesn't feel like her own. She doesn't know why and tries to laugh it off, but she stops abruptly and returns to the console room.

Meanwhile, the Doctor and Ian have come across an extraordinarily large obelisk in the middle of the plain. They can't quite make out the figure which sits on top of it, but they can tell from its decayed state that it is quite old. It is curious, they agree, but definitely too primitive to have any connection to the force which has grounded the TARDIS.

Ian spots a small pool nearby, filled with what he thinks is water. The Doctor stops him just before he puts his hand in, certain that there's more here than meets the eye. At the Doctor's urging, Ian produces the necktie he wears round his waist to hold his pants up(?!). The Doctor dips one end into the pool and soon the tie is smoking. The pool is filled with acid, which the Doctor has just saved Ian from dipping into. His tie is a small loss comparatively.

The Doctor sums up the disparate elements of this puzzle: echoes, interference, gold, and formic acid. Nothing is adding up just yet and he is ready to move on. Ian tosses away the remnants of his Coal Hill School tie and is ready to follow the Doctor, but he is startled by a splash in the acid pool. He does not see the giant ant creature who watches from behind a crag. He thinks there was something in the pool. The Doctor dismisses this grumpily, telling him they must keep their minds on the problem at hand. As they start to move away, though, the high-pitched chirping sound explodes all round them. They can hear it quite clearly now.

In the TARDIS, Barbara can hear it too. It pierces her senses as it did Vicki's and she cringes, covering her ears. This time, Vicki cannot hear it in the quieter confines of the living quarters. Whatever power holds the ship here also can affect other parts of it, for the doors open by themselves. Barbara staggers away into an alcove but is frightened when a piece of equipment jumps and clatters near her. The main console is suddenly spinning in circles where it sits. When it finally stops, Barbara goes to it, but it is no comfort. With the chirping sounds blasting her mind, she is unable to resist when her left arm - the one on which she wears Nero's bracelet - rises up against her will. Her face and mind go blank and Barbara walks zombie-like out the open doors, her arm leading her onward. The doors close behind her.

The Doctor and Ian have not found the source of the noises but they haven't really been looking. They are clearly made by some sentient beings which they are not sure they want to meet. Instead, the two men hurry away, still hoping to find the source of the interference.

The sounds die down and Vicki awakes. She expects to see Barbara but finds herself alone. Going out into the console room, she is frightened to find no one there and the doors standing wide open. She calls out in fear for Barbara.

Vicki's voice carries across the plain to the Doctor and Ian, who hurry back to help her. Ian, not looking where he is going, stumbles into a trap. He is captured in a web-like net which covers him from head to toe. The tendrils of the web sting his skin like thorns. He urges the Doctor to leave him and return to the ship.

Elsewhere, Barbara's arm leads her on, directly toward a deadly acid pool.

Vicki is startled when the deck of the ship starts rocking again and she is thrown onto the console. In her panic, she presses every button and switch she can find. The power returns to the console and the sounds of dematerialisation fill the room. The picture on the scanner lurches and spins crazily.

Barbara is diverted from her path toward the acid pool at the direction of one of the giant ant creatures. Slowly, deliberately, led by her outstretched arm, she skirts the pool and continues forward. The ant does not follow.

The Doctor, agitated and wary, returns to where he left Ian. The web netting is gone and Ian is unconscious on the ground. The Doctor touches him and Ian wakes with a start. All of his nerves are on fire thanks to the stinging web material. Some of the strands still cling to his skin and his face has erupted in red blisters, but otherwise he is unharmed. He brushes off the remaining strands and curse this place. He is more than ready to return to the TARDIS. The Doctor must give him the bad news about the ship.

Elsewhere, Barbara's zombie-like progression is watched by another strange creatures. It is humanoid, with two arms and two legs, but its face is lined with light and dark markings, it sports two antennae on its head, and its eyes are the composite ones of an insect. It also has a sent of intricately-lined wings - like a giant butterfly.

The Doctor and Ian return to the site of the TARDIS' landing. The Doctor tells Ian that he doesn't believe the ship has dematerialised as no one else should be able to enter it or operate the controls. But beyond that he has no further answers. He stops talking suddenly and leans heavily against an outcrop of rock, gasping for breath. He explains that the ADJ's are no good after an hour's use, but he believes they can breathe in the thin atmosphere with some difficulty. Slowly they remove their jackets and adjust to the lack of oxygen. After a few moments of deep breathing, they are ready to move around again. The Doctor spots some furrows in the rocky ground and Ian identifies them as signs of dragging. The TARDIS has been manually moved from here.

Indeed, unknown to them, the ship slides noisily across the hard ground as they speak. But there is nothing manual about its journey. There is nothing visibly acting on the ship at all, but it scoots slowly along. Inside, Vicki tries to keep her balance as the whole of the ship moves beneath her. She sees the barren landscape passing by on the scanner, including one of the giant ant creatures.

The butterfly creature follows Barbara closely. It seems to know the influence she is under and acts accordingly. By changing the direction of her arm, it changes the direction she walks and it steers her unprotesting into a nearby cave. There, two more of the butterflies wait silently. Barbara suddenly starts to turn and leave, drawn by the unknown force, but the butterflies stop her and bring her back. One of them removes the bracelet from her arm and drops it into an acid pool. Slowly, Barbara awakes from her trance. She looks around and must fight back her fear at her new surroundings and the strange creatures. When the butterflies don't answer her questions right away, she tries to bolt from the cave. But the creatures block her way and tell her to stay put. At least they can communicate.

For Ian and the Doctor, a dead end is all they find. The thin soil has given way to bare rock and the shallow tracks they were following have disappeared. The bizarre nature of this world nearly overwhelms the Doctor, but he refuses to give up. Looking round, he finds the tracks of some sort of bipedal creature, but they look more like insect claws than feet. Meanwhile, Ian has found the remains of a small insect creature lying on the ground. It is a well-preserved husk, hollow inside, but with a size and shape that indicates intelligence far above the insects of Earth. The Doctor pronounces it "Menoptra" and seems as though he's very familiar with these creatures. He has never been to it before, but all the clues point to this being the planet Vortis in the Isop galaxy. He has heard of it. However, Vortis has no moon and this planet has several of varying sizes. The Doctor agrees that planets can attract satellites to themselves, but usually only with the advent of some new and powerful force. It seems unlikely to him that Vortis could develop such a force, but it is the only answer so far. The two men move off to continue searching.

The TARDIS continues its slow journey across the planet's surface. It is being dragged toward an opening - a sort of doorway. The walls of the "enclosure" appear to be made of the same webbing which trapped Ian, and two animated tendrils reach out from it toward the incoming object. Vicki watches this from the scanner, a look of shock on her face. Is there no one to help her?

Barbara has decided to bide her time. She speaks with the butterfly creatures, but warily. She wants to see weather they mean her harm or not. She has learned that two are male - Hrhoonda and Hrostar - and one is female - Vrestin. But that is all. They listen as she explains how she and her friends arrived here. When she is finished, Hrhoonda is still suspicious and thinks they should kill Barbara for their own safety. Her existence is of no concern to him or to their mission. Mercifully, the others hold him back. She is an enemy of the Zarbi - the giant ant creatures - they reason. So are they. They should join forces. Barbara offers to help them, these butterfly creatures called Menoptra. Barbara is sent to one side of the cave, watched over by Hrostar, while the others decide her fate. Hrhoonda persists in his viewpoint, saying that Barbara will betray their location, either on purpose or through torture by the Zarbi. For the safety of their mission, she must be destroyed. Vrestin cannot get through to him that she may be a valuable ally and calls Hrostar over to help her make her case. But Barbara is not content to sit and wait for these strangers to decide her fate. Finding a piece of wood near her feet, she manoeuvres it so that Hrostar must step over it to get past her. When he turns and does so, she grabs the wood and lifts it up, tripping Hrostar. She barrels past the others, who are stunned by the sudden movement, and leaves the cave. She trips as she goes, but picks herself up and keeps moving. There is no sign of pursuit.

Ian and the Doctor continue their search for the TARDIS. Ian notices a bright intermittent light on the horizon, similar to the aurora borealis on Earth. The Doctor believes it is not natural, nor is the strange "building" he notes below them. They stand on a ledge overlooking a long complex with a roof and walls stretching toward the horizon. It looks organic but clearly did not form here naturally. They surmise that that is where the TARDIS was taken and they set off for it.

However, they do not get far before they are surrounded by Zarbi - the giant ant creatures - along with the grub-like creature. Their chirping fills the air. The Doctor stands still and observes, but Ian keeps trying to avoid their feelers. The Doctor urges him to stay still and to avoid provoking them. Ian wonders if they can speak to the creatures, but the Doctor knows that is impossible. He has never encountered anything like them before. Shortly, the Zarbi begin moving off, herding the Doctor and Ian with them.

Inside the TARDIS, Vicki is pleased to find that the rocking motion has finally stopped. The ship has stopped moving at last, but the scanner is blank. Suddenly, the doors open and Vicki starts. Thinking that Ian and the Doctor have returned, she calls out to them. When there is no answer, she boldly charges outside, desperate to do something other than stay in the ship. She emerges into a "room" that appears to have been grown more than built. It has four walls and a high ceiling, but they are diaphanous and oddly insubstantial. They are made of black webbing, delicate-looking and intricate. But the walls are solid and enclose the room completely. It is the inside of the structure Ian and the Doctor observed from above. Vicki looks round in stunned silence, but soon the silence is broken by the high-pitched chirping of the Zarbi. Two of the giant ants corner Vicki against the wall. She cries out in terror.

The Doctor and Ian are brought to the structure and directed to an entrance. They marvel at the organic composition of it as well as its size. The Doctor surmises that it took perhaps 200 years to grow to this size. Perhaps this is Vortis after all, but in a future where much has changed. The two men are forced inside.

Vicki is guarded by one of the Zarbi while the other cautiously approaches the TARDIS. It seems anxious to examine its prize. The creature manages to cross the threshold and peer into the console room, however its chirping suddenly pitches higher and faster. It backs out of the ship quickly, as if repelled by something inside. The creature spins crazily and continues chirping in obvious agitation.

Suddenly, an alarm sounds throughout the complex. The Doctor and Ian enter the room and take in the situation immediately. The Doctor finds his ship but its doors are wide open. Ian goes to comfort Vicki, shooing away the hovering insects. He learns that Barbara is not with her and has disappeared. The Doctor is incensed that one of the ant creatures has tried to gain entry to the TARDIS and asks out loud for an explanation of this despicable behaviour. The response to his shouting is the crowding of a group of the creatures around him. The alarm still blares.

Elsewhere, Barbara has stopped running, convinced she is not being followed. But she is far from safe. She has no idea which way to go to find the TARDIS nor where her friends might be since she was under the influence of the Zarbi. She finally decides to pick a direction and go when she is set upon by two Zarbi. They grab her, and as she screams, they toss her to the ground.

Back in the cave, the butterfly-like Menoptra give little thought to Barbara. They clearly have a more pressing agenda. Again a debate is in progress, Vrestin wanting to use their radio transmitter to warn their forces of the "power of the Animus" and Hrhoonda fearing that breaking radio silence will draw the Zarbi to them. Again Hrostar is called in and he sides with Vrestin. Hrhoonda can only watch the cave entrance in trepidation as Vrestin activates the organic crystalline radio set. Despite several attempts, she receives no answer from the Menoptra invasion force. Hrostar is sure they fear detection and will not respond, but agrees to let her continue trying from outside the cave where transmission will be clearer. But it is too late. Hrhoonda shouts a warning that the Zarbi are coming.

Two of the creatures lead Barbara back into the cave. She walks zombie-like, a large gold wishbone-shaped harness around her neck controlling her. Hrhoonda tries to engage the Zarbi with a spar of rock, but he cannot hold them off. Vrestin and Hrostar try to smash the crystal radio in order to keep it out of enemy possession, but are set upon by the second Zarbi. One of the crawling bug creatures - known as a venom grub or a larvae gun - also arrives and the battle is quickly over. The grub shoots at and kills Hrhoonda with a burst of energy. Hrostar urges Vrestin to flee. She does so and he is captured.

Forgotten for a moment in the aftermath of the battle, Hrostar removes the gold harness from Barbara's neck. She comes back to herself with a start, wondering how she got back here and what has happened. Hrostar tells her she was "morphotised" by the harness. It is made of gold and with it they can control anyone. Barbara now realizes that it was Nero's gold bracelet which brought her here in the first place. Hrostar tells her they were spare in order to work as slave labour at a place called the crater of needles. The work is hard and few survive there for very long. As if to underscore this picture of brutality, the Zarbi grab Hrostar and force him to his knees. He knows what is happening and his face is full of fear. Barbara watches in horror as the creatures rip off his beautiful butterfly wings. He has truly lost his freedom now.

Meanwhile, the Doctor tires another tack to communicate with the Zarbi - hand signals. He tries to indicate that the TARDIS is his property, but it is clear that he is not getting through to them. There is some intelligence behind these creatures, clearly, but it is increasingly obvious that the intelligence is not innate to the ants.

Ian and Vicki can see that the Doctor is speaking, but can hear nothing. They must trust that he will be all right.

The Doctor assures the owner of the disembodied voice that they come in peace, but the voice persists in asking questions about his invasion force and its weaponry. The voice believes him to be part of the Menoptra spearhead and demands answers. It will not listen to the Doctor's protestations and vows to make a show of strength, to show him the fate which awaits all the Menoptra ships.

The clear tube retracts into the ceiling and the Doctor is herded over to join Ian and Vicki. A previously unnoticed appendage in the wall comes to life and points at the TARDIS. It is clearly a weapon; however when it fires, there is no damage to the ship. The power of the weapon feeds back and causes a small explosion in the room. The Zarbi scatter, chirping madly, as smoke fills the air. The Doctor is surprised at the result, realizing that the ship's power must be on again to cause such a reaction. He turns to Vicki, who admits that she activated the controls haphazardly. She really doesn't know what she did. The Doctor gleefully calls it a miracle - she has realigned the fluid link and reactivated main power. They could leave the planet now if only Barbara were here. Taking advantage of this turn of events, the Doctor addresses the disembodied voice and offers their assistance.

Outside, on the surface of Vortis, the Menoptra's plans to defeat the Zarbi continue. Vrestin is still free. She has come to the complex and now observes the web-like structure from a high cliff. Unfortunately, she is forced to fly away from his vantage point by the arrival of two Zarbi. She gets away safely.

The Doctor is connected again with the voice via the helmet-like tube. The voice threatens him again and asks to know how the TARDIS withstood its gun. The Doctor asks to know the operating principle of the gun in order to answer. They are in a stalemate, and the voice is certain now that she is not dealing with the simple Menoptra. She changes tack, offering him freedom in exchange for his help in defeating her enemies. The Doctor is informed that Barbara is at the crater of needles "beyond the great web". The voice wants to find out the exact position of the Menoptra fleet, which is hiding from its scanners out in space. Without overtly agreeing, the Doctor tells her he will bring his astral map out of the TARDIS and investigate the situation. But he will need help to do so.

The tube retracts and the Doctor believes he has given the voice something to think about for the moment. He rejoins his friends and tries to take them to the TARDIS to speak in peace. However, the Zarbi hold Vicki back. She bravely agrees to stay here while the other two go into the ship.

Outside, Vrestin turns the tables on the Zarbi and pursues them...at a safe distance of course. They arrive in the web complex and the Menoptra now knows the location of the complex's entrance. She waits outside the lair of her enemies.

While Vicki waits anxiously outside, Ian and the Doctor take a moment to literally catch their breath in the TARDIS. It is much easier to breathe here, out of the planet's thin atmosphere. Ian had hoped to find a message from Barbara, but there is nothing. The Doctor gives him some ointment for the blisters on his face. As he applies it, Ian wonders about their predicament. He agrees with the Doctor that the Zarbi resemble giant ants, a very different evolutionary pattern here than on Earth. Knowing how relentless and destructive a colony of ants can be, Ian is worried. The Doctor is more concerned about the voice, which he dubs the "queen of the ants". She is the power behind the Zarbi. He decides to try and trick her in order to allow Ian to escape and find Barbara. He tells Ian about the "crater of needles", but where that is is still a mystery.

The Doctor gives Ian several small pills which will help with his breathing outside. He is to give some to Barbara when he finds her as well. The Doctor starts to wheel the astral map toward the door. Ian thinks he is helping when he moves to uncouple its cable from the wall socket, but the Doctor stops him sharply. He must never break the time and relative dimension link. Never!

They bring the map device outside and the Doctor busies himself around it. Ian goes to warn Vicki of their plan. The Doctor feigns annoyance that the map will not work and shouts out to be put in contact with the "queen". The tube drops and the Doctor announces that some force of hers is affecting the instruments. If she does not neutralise it, he will be unable to help her locate the invaders. Grudgingly, she agrees to immobilise her force fields near the astral map, but warns him that death awaits him if this is a trick.

Shortly, the lights pulsing in the walls go out and the Zarbi guards go nearly dormant, scratching around on the ground like the earth-dwellers they truly are. Ian approaches one of the Zarbi and waves his hands in front of its eyes. It makes no response and he is satisfied. He hurries off to find Barbara. Vicki is worried for his safety, but the Doctor is confident. Ian is "very good at this sort of thing". He'll be back.

But Ian is having trouble even getting out of the complex. It is organized like an insect burrow, its walls and corridors mazelike and identical. Added to that, the Zarbi are still very much active in other parts of the complex. Ian must try to remember the way he came in, retrace his steps, and evade the patrolling Zarbi.

Meanwhile, the Doctor takes advantage of the peace and quiet to check his astral map to see if he can learn more about the Menoptra invasion. Soon enough, he picks up their radio signals. The "queen" was right, they are massing not far away. He find tunes the instruments and has Vicki activate a recording device. He learns of a planned landing en masse on the Sayo Plateau just north of the crater of needles. It appears to be the centre of all activity here on Vortis.

Ian continues his escape attempt, still unnoticed for the moment. However, his luck runs out as he nears the exit. He emerges from a passageway to find a Zarbi blocking his way. It faces away from him and so does not notice him right away. But it turns round just as Ian attempts to sneak past and it advances on him, chirping rapidly. Ian wrestles with the giant creature but it is very strong. He finally manages to flip it over on its back, hoping that this will give him enough time to get out. But an alarm sounds almost immediately and a wall snaps shut ahead of him, trapping him in a sort of cage.

The alarm also sounds around the Doctor and Vicki, and the room comes to life again. They lights pulse and the Zarbi come back to life, chirping madly. The Doctor tries to deny it, but Vicki is certain that Ian's been caught. They are menaced by a Zarbi.

A group of Zarbi race through the corridors to find their captive still in its "cage". They bring forward one of their venom grubs to kill him. However, Ian manages to side-step the blast, which goes through the web walls and blows open the swinging wall on the other side. The "cage" opens with a burst of smoke and Ian runs for his life. He manages to make it outside, but no sooner has he gained his freedom than he sees a strange winged creature flying toward him. Unaware of the Menoptra, Ian runs away.

The Zarbi have morphotised Vicki with one of the gold harnesses and the Doctor has been summoned to speak to the irate "queen" once again. She demands to know why Ian has attempted to escape. The Doctor puts her off with some very literal wordplay, angering her. She threatens to kill the Doctor and Vicki for Ian's escape, but the Doctor warns her that his information will die with him if she does so. The queen thinks he is lying, but he dares her to call his bluff. The communication tube retracts and he knows he has bought them some more time.

Returning to the astral map, the Doctor directs the Zarbi guard to remove the harness from Vicki's neck. It does so, but not likely due to the Doctor's request. Vicki is released from her zombie state and collapses into the Doctor's arms. He reassures her that all is well for the moment and then tries to look busy at the astral map. He sends Vicki into the ship to fetch a red box with a recording compound in it and gives her its location.

However, when Vicki returns with the box, the Doctor is angry that she has brought the wrong thing. He distinctly said a white box! The red box contains only a glass-encased spider carcass, one of his specimens. It is useless and he sends her back into the TARDIS. Vicki goes back, looking at the dead spider with some morbid interest as she goes. But when the Zarbi guard catches sight of the spider, it backs away, seeming to fear the harmless creature. It is a revelation.

Ian, meanwhile, has made an uneasy truce with the Menoptra who has chased him. He has learned her name and that she is the only escapee from the Menoptra spearhead. He knows she has seen Barbara. Vrestin and Ian watch the complex from a high vantage point. Ian rests and catches his breath before heading off to the crater of needles. Vrestin, unable to cope with the strange-sounding human names, calls him "Heron". She explains to "Heron" that the crater of needles is a dangerous place, where many of her friends, now wingless, are held captive. Ian thinks this only natural treatment since the Menoptra are trying to invade the planet. But Vrestin tells him that Vortis belongs to them - they were driven from it and now return to reclaim it for themselves.

Ian is sceptical, but when Vrestin accurately describes the behaviour of the Zarbi - now returning to the complex after a brief search - he has more respect. She begins to tell him their story. The Zarbi are actually unintelligent scavengers, harmless but essential to the life pattern here. But with the arrival of the Animus - what Vrestin calls "the dark power" - the Zarbi were given a guiding intelligence and became militant. This is the "queen" with which the Doctor has spoken. The web complex - called the Khazenome - appeared soon after and began to spread like a fungus across the planet. The Menoptra were forced to flee the planet or die, as they were a peaceful people with no weapons. At the same time, the new moons appeared, drawn by the immense power of the Animus. It is on one of those dismal satellites that the Menoptra have lived, barely surviving. Now they are planning to attack Vortis, not because they are ready, but because they must. The Khazenome grows daily. If it encircles the planet completely, it will be too late to reclaim Vortis. Ian believes her and suggests they join forces. She can lead him to the crater and he can help free her friends along with Barbara. Vrestin agrees and they set off.

They make good progress on foot, but the crater is far away. Ian cannot fly, and even his walking speed is impaired by the thing atmosphere. It will take another two hours to reach the crater. Ian is ready to press on, but they are forced to stop when a group of Zarbi emerge almost from nowhere and attack. Ian and Vrestin try to run but are stopped by a steep drop in the terrain. Vrestin will not leave Ian, even though she could fly to safety. Together they head for a fissure in the nearby rock wall. It is barely wide enough to admit them, but the Zarbi should not be able to fit in.

Ian and Vrestin fall quite a long way in a hail of dust, but luckily land on a patch of soft sandy ground. Both are unharmed. They gather themselves, choking from the dust, and look around. They are in an underground cave - newly opened to the sky far above. The walls are dark and there is little light inside. On one wall is a primitive drawing. But Ian has no time to study it as he and Vrestin are suddenly surrounded by a group of creatures, armed with spars of rock.

At the crater of needles, a group of Menoptra - all stripped of their wings - labour under the control of the Zarbi. They "harvest" heaping mounds of seaweed-like vegetation by hand, then carry it to a large acid pool and drop it in. Then they return and continue. Among the group is Barbara, who struggles to continue after long hours of toil. She is breathing quickly and shallowly due to the thin atmosphere. Her eyes ache and she cannot see clearly anymore. Hrostar is with her, and encourages her to sit and rest for a moment. He will keep watch. In her distress, Barbara also realizes Hrostar's pain and asks if his wings have healed. If they still hurt him, he will not say. But his distress over never being able to fly again is very deep and clear.

Their conversation is interrupted by the arrival of a Zarbi, who prods them into continuing to heap the vegetation into the acid pool. It leaves when they start again. Hrostar explains that they are providing the raw material for the growth of the Khazenome, of which he has told her before. The vegetation is broken down by the acid and drawn through underground streams to a place called "the Centre". It is from there that the Khazenome grows outward to encircle Vortis. The Animus lives in the Centre, but no one has seen it and lived to speak of it.

Another Zarbi comes to prod them back to harvesting. They get up and move slowly, Hrostar explaining the true nature of the Zarbi - unintelligent scavengers now directed by the Animus. He also reveals to Barbara more about the group of Menoptra she met. The three of them were all that was left of a scout party who arrived in advance of the invasion spearhead. The weapons they carried proved useless against the Zarbi and it was all they could do to get away with the communicator. And even that proved useless - the spearhead is coming soon with no knowledge of the strength of the Zarbi. If they could just get past the Zarbi and into the Centre, they have a new weapon called the Isop-tope which they believe will destroy the Animus. It is untested, but it is their only hope.

Inside the Khazenome, the Doctor bides his time, hoping for Ian's safe return. He watches with interest as the Zarbi come and go. One passes by clutching an instrument in its mandibles; he does not know that it is a captured Menoptra weapon. Vicki joins him, fresh from hiding the spider specimen for safe keeping. She wonders why they don't use the Zarbi's fear of the spider to escape right now. The Doctor believes it will prove more useful later, once Ian and Barbara have returned.

Suddenly, the area is abuzz with activity. A Zarbi approaches with one of the gold harnesses, heading straight for Vicki. She tries to run - to reach the astral map and the spider hidden there - but she is stopped by a venom grub directly in her path. The harness descends and she is immobilized. The Doctor is forced to stand beneath the communications tube again. The Animus is angry, certain that the Doctor is wasting time and trying to save his own life. She threatens to kill Vicki to force his obedience. To save her, he is forced to reveal that the Menoptra are massing on the nearby planet Pictos, heading straight for Vortis. If she wastes time on vengeance, the Menoptra will annihilate her and her Zarbi. She wants to know where they will land. The Doctor acts as if he does not know, certain she will spare them until he finds out. She sends him off to find out, grudgingly agreeing to release Vicki.

The tube retracts and a Zarbi removes Vicki's harness. The Doctor comforts Vicki, telling her what has happened. Their usefulness to the Animus is nearly at an end and he is clearly worried. He removes the recorder from the astral map, knowing it contains the Menoptra landing site, and puts it in his pocket. Suddenly, another alarm blares and the Zarbi scurry around, receiving instructions. The Doctor believes it is the Animus putting her troops on alert for the impending invasion. He is surprised she has acted so quickly on such little information. He is concerned that he and Vicki must leave here, soon. He sends her off to the TARDIS to fetch his walking stick, disturbed by all the activity.

The same alarm and activity is talking place at the crater of needles as well. The Zarbi move agitatedly, apparently receiving instructions. They begin herding their prisoners together. The work stops and they are moved into a makeshift cage.

Far down below the ground, Ian and Vrestin are having a difficult time in getting through to their captors. The creatures are small and sluglike, with huge insect eyes for seeing in the dim light. Their arms and legs are stubby and stunted - they move with hops and shuffles. Their backs are hunched and covered with a type of carapace. They speak brokenly with guttural tones. They are primitive underground dwellers. They believe that Ian and Vrestin invaded their lair from above and cannot seem to understand the accident that truly brought them here. It is clear that the prisoners are more mobile than their guards, so to shorten the odds, Ian and Vrestin are forced to dip their hands into a font of organic liquid adhesive. Their hands are glued together.

Thus bound, the prisoners are at last addressed by the leader of the underground creatures. His name is Hetra. He tells them that his people - the Optera - know only of hate and death from above. There is "liquid death" and "the creeping destroyer" and creatures which do not stand upright. True, Ian and Vrestin do not appear connected to any of these things, but they did come down from above. Hetra plans to consult the "chasm of light" to determine their fate. If they are found to mean harm to the Optera, they will be killed.

In their cage at the crater, Hrostar and Barbara have befriended two fellow captives - Prapillus and Hlynia. All are wondering why the work has stopped and why the Zarbi appear so agitated. Prapillus believes it is the coming of the invasion force and Hrostar agrees. Hlynia is overjoyed at this, thinking their exile from Vortis will soon be over, but Hrostar urges caution. How have the Zarbi been alerted to the imminent arrival of the invasion force? He was certain their plans were secret. Taking a leap of logic, Hrostar wonders if Barbara's friend the Doctor - the brilliant man of science - could be helping the Zarbi. Barbara says she doesn't believe so, but her tone betrays uncertainty. However, she reasons that the Zarbi are on alert everywhere, not just here. It's just coincidence that the Sayo Plateau, their landing site, is here. But that is immaterial to Hrostar. For whatever reason, the Zarbi are alerted here. The invasion force will be met and massacred.

Barbara must be reminded that the Menoptra's weapons have already proven useless against the Zarbi, and that the invasion force doesn't know this. The plan was to land in secret and overwhelm the Zarbi and the Animus by surprise and force of numbers. A suicide attack. There is no backup plan - there was not time. The invasion had to come now, ready or not. Failure in this means failure forever. Their desperation rising, Prapillus is first to voice the obvious: they must somehow warn the spearhead. If they can climb up to the Plateau and be there when the first Menoptra arrive, they may be able to turn the others back before it's too late. Hrostar is nervous, noting the larvae gun pointing at the cage door, but Barbara joins with the others and tells him they must try. With the Zarbi on alert elsewhere, there are few here presently. If they can disable the gun, they may have a chance. Prapillus has a plan for that and starts to knock and opening in the side of the cage.

Back in the Khazenome, things have calmed down a bit. Only one Zarbi now guards the Doctor and Vicki, the others engaged elsewhere. Vicki has produced the spider specimen and uses it to distract the Zarbi. The Doctor uses his walking stick to slide one of the gold harnesses to him without touching it. It has been forgotten on the floor. He examines it, explaining to Vicki the incident with Ian's gold pen. There is some influence on this planet which interacts with gold to control it. He must find an equal and opposite influence for his plan to work. Then, with a look of realisation on his face, he notices his hand...

Prapillus is now free, unseen by the Zarbi or the larvae gun. Indeed, his friends do not know where he is either. However, they are poised to burst out of the cage and attack the larvae gun. When a clatter of stones draws a Zarbi's attention away, they know it is time to act. Barbara and Hlynia go for the gun, while Hrostar helps Prapillus disable the Zarbi, turning it on its side helplessly. The men join Barbara and Hlynia, Prapillus taking the lead. He hoists the larvae gun up and smashes it underside first onto a rock wall. The creature dies gruesomely. Barbara is shocked, not realising it was a living thing, but she has little time to worry. The group hurries to the Plateau.

The Doctor has managed to attach the gold harness to the top of the astral map without arousing suspicion. He has made it so that the TARDIS' energy is opposite to that of the Animus, but is it stronger? He activates two switches on the astral map and there is a short and a small explosion. The Doctor reaches up to touch the harness, seemingly pleased with the results. However, the explosion has drawn the attention of the Animus, and while the Zarbi holds Vicki, the Doctor goes to the communications tube.

Again the Animus is angry, demanding the Doctor's report on the Menoptra landing site. He tries to pass off the explosion as a system failure in the astral map, a damaged part which will delay his investigation. He even holds up the recorder to pose as the damaged part. However, he accidentally activates it as he does so, and cannot shut it off before the Sayo Plateau landing site is revealed. The Animus realises the Doctor has had this information all along and promises to "deal with him" as soon as the invasion is repelled. Vicki, already wearing a harness, is placed against one wall. The Doctor is herded there as well and a harness placed on his neck.

Hrostar and Hlynia reach the foot of the Plateau, helping Barbara up. The climb is hard for her in the thin atmosphere. There is still no sign of the invasion spearhead. However, Prapillus joins them and reports an influx of Zarbi now surrounding the Plateau. There can be no doubt that they know the spearhead is coming here. And there is no other explanation for Hrostar other than betrayal by this Doctor.

In the underground cavern, the trial of the prisoners is incredibly brief. Every creature who has come to their domain has come to prey on the Optera. These 2 are no different and must die. They are to be thrown into the chasm of fire. Ian tries a desperate gamble to save them, putting forth Vrestin as one of their own kind. Hetra does not believe him, painting a picture of the surface world as a wilderness of predators and death. The light blinds, the creatures kill, and no Optera who has ever gone up has returned. Those who come from above only do so to kill. Vrestin rebuts, telling them that the surface belongs to them - that they lived their in peace before the coming of the dark power of the Animus. They have been underground so long they have forgotten. But she and Ian are part of a group trying to purge Vortis of the Animus so they can all be free again. Ian understands - these "Optera" are really Menoptra devolved underground for generations.

The creatures are stunned, pointing to more cave drawings on the walls. They are primitive renderings of flying creatures. Menoptra - the gods of the Optera. Vrestin reiterates that they are kin, not gods, to the Optera and that the wretched underground dwellers were born to freedom, peace, beauty, and light on the surface. Hetra is beginning to believe, but knows nothing but death on the surface. Vrestin urges him to join with her and Ian to fight the Animus and regain their birthright. To prove who she is, Vrestin unfolds her magnificent wings. The Optera bow in homage to her.

Barbara, Prapillus, and Hrostar reach the top of the Plateau just in time. The first wave of the spearhead begins to land. One of the leaders lands near to Barbara and Hrostar - exchanging the code word "electron" to establish identities. Hrostar explains what happened to the pilot party and their ineffectual weapons. He urges the leader to call off the landing to avoid a massacre. But the leader sadly reports it is too late.

Barbara spots an opening in the Zarbi group surrounding them and she takes it. She and Hrostar hook up with Prapillus and keep running. However, a dead end awaits against a tall black rock formation. Barbara stumbles as she approaches and leans on the rock for support. Part of it gives way - a secret door in the rock. She and her friends hurry through it, closing it behind them just as the Zarbi catch up. They are safe.

Back in the Khazenome, the Zarbi seem absorbed in receiving instructions. Vicki cautiously opens her eyes and, making sure she's not seen, removes her harness. She uses it to remove the Doctor's without touching it. He comes back to himself slowly, taking in the situation. Vicki explains that she herself put the harness on, not the Zarbi, and it was one that the Doctor converted with the astral map. She only pretended to be morphotised. The Doctor believes that his plan has worked; if the gold harness is no longer under the control of the Animus, he himself should be able to control it as he had planned, with the power of his ring.

They are forced to curtail their conversation and pretend to be morphotised as the two Zarbi finish receiving instructions and turn around. One of them leaves and there is only one to guard the prisoners. The Doctor takes the converted harness from Vicki as she pretends to awake, crying out loudly. This attracts the Zarbi, who notes the harness on the floor. As it bends to examine the harness, the Doctor places the converted harness around the creature's torso and thrusts his ring into its field of vision. The Zarbi is now under the Doctor's control, following the movements of his hand.

Wasting no time, the Doctor and Vicki leave the room, bringing their Zarbi with them. They seem able to move freely through the corridors. Vicki is concerned at the seeming lack of Zarbi about and the Doctor reasons that most have gone to the Plateau. However, he doesn't think for a moment that will make their escape easy. They move cautiously forward.

The passageway that Barbara found leads to a decorated, high-ceilinged chamber. Barbara proclaims it beautiful as Prapillus explains it is a "temple of light". It was a place of worship for the ancestors of the Menoptra, and there were many on Vortis long ago. They were all believed to have been lost. Hrostar has been keeping watch at the entrance and reports no further sounds from outside. Barbara thinks they should wait a while longer to make sure it's safe.

Prapillus is startled to see another Menoptra face looking back at him from within the temple. The stranger identifies himself as spearhead leader Hilio and presents the proper code word. Hlynia also appears, vouching for Hilio. They are all glad to see the others still alive. Hlynia explains there is another entrance to the temple elsewhere on the Plateau and that is how they escaped. The Menoptra lament the failure of the spearhead, but Hilio will not hear of defeat. He plans to go back out onto the Plateau and try to regroup his forces. Ever the realist, Barbara points out that there likely isn't many of his forces left alive. Hilio is angry at this talk and demand to know who she is. Even with Hrostar's backing, he doesn't trust her. Barbara tells him bluntly that he doesn't have much choice.

Hrostar confirms Barbara's grim assessment of the spearhead, and Hilio is slowly convinced. Without contact from the spearhead, the main force will also land at the Plateau and will be similarly massacred. Prapillus, older and more thoughtful than the others, explains that the Menoptra have no aptitude for war. They have known only of peace and beauty, temple making and carefree flight. They knew nothing of enemies of weapons until the Animus came and forced them from their home. However, Prapillus believes that his captivity at the crater has taught him of strategy and resilience. He nearly gave up when his wings were taken from him, but he took a lesson from their gods of light and used his mind to forced himself to survive. Barbara has never had wings and they can learn much from her about how to confront danger rather than to fly away from it. He urges the others to listen to Barbara.

Put on the spot, Barbara must think fast. She finds out that the original plan - had the spearhead been victorious - was to head for the Centre and attack the Animus. Hilio produces the living cell destructor, their ultimate weapon. It is small but it is powered by the isop-tope and is designed to attack the Animus and her web by reversing the growth pattern of its cells. They would grow inward and die. Hlynia fears that it will be as useless as their electron guns, but it appears to be their only hope. Waiting will gain them nothing and Hrostar points out that their only hope for success is action. The small but determined band will continue with the spearhead's mission to win their planet back.

Far below ground, Ian and Vrestin have been released from captivity by the Optera. The underground creatures have pledged to help defeat the Animus, but Vrestin is unsure that the fearful creatures will be of much assistance in a fight. Ian, too, is worried, but their help is better than going it alone. Hetra joins them and explains that the underground tunnels are dangerous, but they lead to the Centre. There lies the root of all evil. Hetra calls it Poiderrach, but Vrestin knows it as the Animus. Hetra agrees to take Ian and Vrestin to the Centre, placing his second Nemini in the lead of the expedition. Cautiously, the group sets out.

The Doctor and Vicki emerge from the Khazenome into the thin air of Vortis, along with their Zarbi guide. There are no other Zarbi about and all is quiet. They want to go to the Sayo Plateau and try to meet up with these Menoptra, but they do not know where it is. However, they spot a worn path of Zarbi tracks in the dust and set out to follow them, nearly forgetting their Zarbi in the process!

The path is long and the thin air takes its toll on the Doctor. The group stops for a rest and Vicki pets the Zarbi. She announces that its name is "Zombo" and that she's quite fond of it. The Doctor, tired and cranky, is not amused by this. It is not a pet and is vital to their survival here. Vicki chides him for being so close-minded, making the Doctor laugh. He promises to look for a collar for Zombo on the way. They continue on toward the Sayo Plateau.

Also on a long trek, the Optera lead Ian and Vrestin onward. They reach a place where stalactites and stalagmites choke the tunnel. The Optera attack the "teeth of stone" with their clubs to clear their path. Ian notices also that the air is growing foul. A thin vapour leaks out from crevices all around. The Optera choose a tunnel that looks somewhat clearer to ease the way. The path however leads to a blank wall and the Optera propose to dig through it to go on. Vrestin goes back to check the other path but is stuck when some rocks in the ceiling start to give way. Ian returns to help her out, but cannot stop the ceiling from caving in. There is no choice but to go forward. But they must break through the wall quickly - there is no air entering the closed-off tunnel and the choking vapour is increasing all the time.

Hilio has drawn a map of the Centre and its entry points through the Khazenome. Barbara comes up with a plan to stage a mock attack at the south entrance to draw off the Zarbi while one of them enters from the north. Hilio, still suspicious of Barbara, dismisses her plan. He suggests that he fly in alone and try to break into the Khazenome from above - he will encounter no Zarbi there. But Hrostar tells him the Khazenome web is too strong. It would grow back together before he could break through it. This leaves them with Barbara's plan, imperfect as it is.

Hilio then volunteers to be the lone attacker on the north. Prapillus challenges him, saying he will go. The 2 engage in an insect challenge rite, but Hlynia interrupts them when she hears a noise outside. There is no time to hide. Prapillus knows that the secret entrance should not open to Zarbi, but it is a Zarbi which enters. The Menoptra are terrified and move to attack. However, the Doctor and Vicki enter behind and call for them to stop. Barbara is surprised and happy to see them, as they are to see her. All are relieved. The Doctor's only question for the moment: where's Ian?

At that moment, Ian is relieved to be breathing more easily. Hetra has opened a small hole in the wall, letting in better air and clearing the choking vapour. Nemini takes her turn next, but does not dig very far before she shouts a fearful warning - the liquid death! Ian knows it is the acid from the pools above and tries to get to Nemini to pull her away. But Hetra stops him and watches as Nemini forces herself into the hole to block the acid. She dies as she does so, but stops the flow and saves the others. Ian and Vrestin are appalled at the sacrifice, but Hetra assures them it was the only way. It is a danger they face daily. The Optera say a silent prayer for their fallen comrade, then move to another part of the wall to start again. Ian looks sadly over Nemini's body, an unreadable expression on his face. Soon Vrestin calls to him that they have broken through quite easily. Their journey must continue.

Barbara and the Menoptra have filled the Doctor in on all that has happened - the invasion, their plans to destroy the Animus, everything. The Doctor agrees that their new plan should work, but he wishes he knew more about the Centre and the creature who lives there. But the Menoptra are little help - there is no historical record of the arrival of the Animus from space, only legends that begin when the Khazenome had already started to grow. Prapillus does reveal that the Centre is at the magnetic pole of the planet, and he and the Doctor deduce from this that the Animus gets its power from the planet's magnetism. That is why the new moons were attracted here, as well as the TARDIS.

The Doctor agrees to the plan of mock attack, putting Barbara in charge of it. However, he suggests that he and Vicki should take the isop-tope and go into the Khazenome from the north. Hilio, still distrustful of these strangers, protests. He does not relish the idea of putting the last hope of the Menoptra people into the Doctor's hands. Hrostar and Prapillus confer on this point, and again the wise Prapillus speaks the words of reason: the Doctor is the first person ever to escape the Khazenome alive. He should be able to get in again safely. Grudgingly, Hilio hands over the weapon. The Doctor praises the Menoptra for all their wisdom.

Finalising plans with Barbara, the Doctor gets ready to leave. But first he must comfort a nervous Vicki. Then he is called over to Prapillus to listen to an idea. Prapillus believes that the tame Zarbi would be of use to them in their mock attack. He has studied the creatures and believes he can make good use of this one. But the Doctor knows that to allow this would mean parting with his precious ring. He says it is of "untold value", and Prapillus reminds him gently of the unique value of the isop-tope they have entrusted to him. The Doctor bristles at this and Barbara joins the conversation before it gets out of hand. She sides with Prapillus. The Doctor hands over the ring with undisguised anger. It is the only way he can express his fear at parting with his ring. Prapillus eyes it curiously, wondering about its secrets. The Doctor denies him the answer to his question and orders him to return in when finished, "upon his life".

The Doctor and Vicki's return trip to the Khazenome is uneventful. They reach the north entrance and see no Zarbi about, but both are trepidatious. The Doctor passes the cell destructor to Vicki, thinking it will be less conspicuous in her possession. Before they enter the control room however, the silence is broken and a group of dormant Zarbi come to life, chirping wildly and surrounding the intruders.

Ian, Vrestin, and the remaining Optera have reached the tunnel directly below the Centre. They must now break through the ceiling above them to reach it.

Meanwhile, Barbara and her party have arrived at an overlook above the south entrance to the Khazenome. Prapillus and the others await Barbara's signal to launch their mock attack. Barbara decides to wait, to give the Doctor enough time to reach the control area.

This he has done already, though. He and Vicki are shoved roughly into the control room by their Zarbi guards. The creatures are much more aggressive than usual and both fear for their lives. Tossed to the floor, they cling to each other in fear. Slowly, a gun-like protrusion comes to life on one wall, twisting to point at the captives. It fires and Vicki cries out. She is silenced down to a whimper, joined by the Doctor.

The Doctor and Vicki are stunned, weakened by the attack, but are otherwise unharmed. They have been kept alive for a purpose. As the communications tube descends toward the Doctor, still kneeling on the floor, Vicki crawls away slowly toward the astral map. The Animus is clearly very angry with the Doctor for his escape. And this time she has no further need for him. The Doctor tries valiantly to fight, decrying her subjugation of both the Zarbi and the Menoptra. But the Animus is out of patience. The only thing of value the Doctor and Vicki have left is "the way they think". Her voice reverberating painfully in his head, the Doctor hears her say they will be brought to the Centre. To her.

Outside the Khazenome, Barbara decides it is time for their mock attack to begin. She is unaware of the Doctor's plight and starts slowly, sending the tame Zarbi toward the entrance.

The Doctor and Vicki are lead down a corridor toward the Centre. He asks Vicki to hand him the web destructing isop-tope, but she reports sadly that she doesn't have it. She hid it in the astral map thinking it would be safe there. She could not have known they would be taken to the Centre so abruptly and now they are without their only weapon.

Seeing only one Zarbi guard at the entrance, the Menoptra draw it out and set the tame Zarbi on it. But it is quickly apparent that all this will do is throw the control harness off of "Zombo". Barbara is encouraged to attack the Zarbi guard, which she does, spinning it round. It goes after her, but is distracted by calls from the other Menoptra. With so many Menoptra calling and moving around, the creature is confused and overwhelmed. Hlynia finally tricks it into re-entering the Khazenome and it simply continues to enter, disappearing inside. The Menoptra know it will return with reinforcements, but that is what they want. The group heads inside to the corridors of the Khazenome, hoping to make detection more difficult in an enclosed space. Hrostar volunteers to attack the first larvae gun they know will be waiting for them. Hilio challenges him on this once again, but eventually relents.

Far below ground, Ian is ready to start the climb up to the Centre. But he is delayed when a liquid begins pouring from the web-choked wall. He thinks it is acid again, but it is actually water. Vrestin explains that all the surface water was drawn deep underground with the coming of the Animus. Once it is destroyed, they can drink and grow plants on the surface once again. Hetra is hesitant to begin the climb, fearing the "blinding heart of Poiderrach". Ian is in favour of leaving the nervous Optera behind while he and Vrestin climb, but she will not hear of it. She exhorts "her people" to believe that they are stronger than the evil of the Animus and that they will regain the goodness of light on the surface. Hetra agrees to join them, but the other Optera elect to remain below. If he survives, he will return to them. Thus settled, the climb begins.

Hrostar has detected the approach of a larvae gun and has huddled everyone together in an alcove. Barbara is anxious to keep going, but stays put as instructed. As soon as the larvae gun comes into view, Hrostar pulls the gold harness off "Zombo" and attacks the gun. He twists the harness into a smaller shape and tries to place it onto the snout of the gun, hoping to control it. But the creature does not respond and it fires directly at Hrostar. He dies, but in so doing, falls backward onto the larvae gun, crushing it. He Zarbi race away, the formerly tame "Zombo" with them.

Hlynia grieves for Hrostar's death, but Prapillus is more pragmatic. They must now succeed, he says, so that Hrostar's sacrifice is not in vain. They have work to do. He and Hlynia move off, but Hilio does not go. Watched by Barbara, he reaches under Hrostar's body to retrieve the harness. It is clear in his voice that he has resented Hrostar's usurpation of his leadership in this expedition. Hilio announces grandly that the next larvae gun is his. He moves off, followed by a pensive Barbara.

The Doctor and Vicki at last reach the Centre. It is enclosed by two diaphanous wing-like doors and beyond it is the shadow of something alive and moving. The doors snap open and the two are shoved inside by the Zarbi. Immediately a bright light flares up from the middle of the room, so bright that neither of them can clearly see its source. The malevolent voice of the Animus booms out all around them "What Vortis is, I am. What you are, I will become." The Doctor collapses, his senses overwhelmed by the assault. He finds himself lying in a tangle of tendrils. It is not webbing, but the very body of the Animus.

Vicki, recovering somewhat, yells at the Animus to leave them alone, calling her a parasite. The Animus does not accept this analogy, believing herself to be an unstoppable power absorbing energy, territory, riches, and culture. All of it now belongs to her. She is revealed to be something like a giant spider hanging from the ceiling of the Khazenome. The multitude of tendrils sinking into Vortis are her very own appendages. She is suspended over a sort of light well that goes straight down. It pulses brightly as she draws energy from it. She has had enough of Vicki's puny insolence and orders her forward along with the Doctor. Her voice is seductive as it beckons them toward her.

Ian, Hetra, and Vrestin continue their climb. It seems endless and they are all very tired. Ian, in the lead, starts to go on, but part of the wall gives way, nearly hitting Vrestin as it falls. Ian catches her and she is fine. She suggests stopping to rest, but Hetra says the Centre is not far and Ian will not stop.

Barbara's party has penetrated deeper into the Khazenome, so far unchallenged. Barbara is surprised to look into one of the rooms and see the TARDIS standing there. Unfortunately, it is being guarded by a Zarbi. Realising that there is no larvae gun, Hilio rashly enters the room, brandishing the gold harness. Barbara and Prapillus are appalled at his lack of caution and planning. Hilio nearly pays the price for his rashness as the Zarbi turns quickly on him and knocks the harness out of his hands. He is helpless. Luckily, Barbara and Prapillus are there to retrieve the harness and subdue the Zarbi. There is no time, though, for recriminations. They look around this main area, trying to figure out how the Zarbi are controlled. They are quick to notice the pulsing lighted wall.

In the Center, things have gone from bad to worse for the Doctor and Vicki. Both are now fully entwined in the Animus' tendrils. The Doctor is barely conscious and mumbling incoherently. He has been dragged much closer to the Animus. Vicki has tried to struggle, but she is now too weak to continue. She collapses beside the Doctor, lamenting her mistake which brought them here unarmed. The Animus is triumphal. Already she is absorbing knowledge from Vicki and the Doctor. Knowledge of the worlds beyond the Isop Galaxy. She will use their knowledge to break free of Vortis and go to Earth. From the human race, she will obtain mastery of space and then no world will be safe from her.

Hlynia has deduced that the lighted wall section is used to control the Zarbi and pass on instructions. Her hatred of the Animus suddenly bursts forth as she grabs a spar of rock and attacks the wall despite her comrades' pleas to stop.

An alarm sounds and the Animus knows immediately what has occurred. She seems slightly afraid when she realises that "aliens" are now in control of part of her great web.

Indeed, the control room seems to be paralysed and lifeless. In the quiet, Barbara looks around. Her eyes quickly alight upon the astral map. Unaware of all that the Doctor has been up to, she is surprised to find it outside the ship. Prapillus wants to use it to contact the main invasion force and turn them back. Barbara tries to properly set the controls to align with Pictos, then Prapillus calls out to the main force, warning them of what has gone wrong. However, there is no reply. Barbara thinks she may have mis-set the device and examines it further. She accidentally knocks one of the panels aside and discovers the isop-tope, which Vicki hid there. All of them are horrified, but Barbara voices the obvious answer: the Doctor has been captured. Hilio takes possession of the weapon and vows to get to the Centre and use the isop-tope himself. The rest of the group follows quickly.

Ian crawls along a narrow tunnel on his back, parallel now to the floor of the Centre. The "ground" above him is soft and spongy but incredibly resilient. He is having a difficult time finding a place to break through. Frustrated, he grabs the sharpened stone spur that Hetra carries and uses it to attack the spongy ground. Hetra does not think he will be able to get through, but Vrestin urges him to keep trying.

As Hilio leads his group through the corridors of the Khazenome, he instructs them all in the use of the isop-topeit must be aimed at the "dark side" of the Animus to work. Suddenly they all stop as Prapillus detects Zarbi ahead. But they do not have the option of trying to find another way around; they must rush the creatures. It only needs one of them to get through with the cell destructing isop-tope. However, with the Animus occupied on several fronts, her energy is divided and her Zarbi are slow to react to the intrusion. By dodging, distraction, and tossing the isop-tope from person to person, all 4 of them manage to get through and reach the doors of the Centre.

They are surprised when the Zarbi do not follow, but they have no time to wonder why. Hilio retakes the isop-tope and charges into the Centre as the wing-like doors open. The others follow behind, into the lair of the Animus, but all the Menoptra are blinded into helplessness by the intense light the creature admits. The door closes behind them and they are all trapped.

Hilio is unable to move, but Hlynia is drawn toward the light. Prapillus is occupied trying to hold her back. Hilio gives the isop-tope to Barbara. She too is dazzled by the light but manages to shade her eyes and so is free to move around. The Animus is thrilled that all her enemies are now in one place, easier to destroy and absorb. On the floor, covered in tendrils, the Doctor is still unconscious. Vicki is barely aware of what is happening but sees Barbara and is overjoyed. It is clear that her energy is fading fast.

Barbara fights the sensory overload created by the light and the presence of the evil Animus. She beckons Barbara to approach so that she may absorb her mind. Barbara points the isop-tope at the darkest part of the Animus, barely able to perceive it in the light, and activates the weapon. Nothing happens and she is certain that the weapon does not work. But she keeps at it, working to find the proper target.

Suddenly, Ian bursts through the tendrils from below. His presence seems to steady Barbara for a moment and she fires the weapon again. The Animus seems unconcerned, pleased that Ian is now here as well. All her enemies in one place. However, it appears that the isop-tope may well be working after all. A sound like a heartbeat becomes audible in the chamber. As the Animus talks of victory and success, Barbara adjusts her aim again. The heartbeat sound becomes more rapid and the Animus' voice beings to slur. Barbara falls to her knees but doesn't drop the isop-tope, keeping it aimed at the Animus' dark side. Soon the heartbeat is one thundering noise and the spider-like creature twitches violently. Her rantings turn incoherent and smoke pours from the light well beneath her. The heartbeat and the evil voice then fade away to nothing and the intense light dies. Just as the Animus dies.

All of the Animus' victims recover quickly. Vicki is near tears as she realises the evil creature is now dead. Even the Doctor begins to stir. They have won.

Some time later, Vortis begins to revert to its former state. The Khazenome is gone, crumbled into the surface dust, and the Zarbi are once again docile scavengers. One of them senses something below ground and digs at it. A stream of liquid sprays up from the ground, pooling nearby. But it is not acid. Hilio and Hlynia drive off the curious Zarbi gathering around the small pool and discover that the liquid is water, finally returning to the surface after the death of the Animus. Hlynia, ecstatic, calls Vicki over to see the miracle. Hilio is certain that with clean water returning to the surface, crops can flourish once again. Vicki calls to Barbara to look. Barbara is playing with one of the now-docile larvae guns as she would with a cat. The creature is quite playful, but Barbara breaks off to go and see the new spring.

Elsewhere, Vrestin is welcoming a group of Optera to the surface for the first time in generations. They are having some difficulty adjusting to the bright light and hide their eyes against its glare. She urges them to be strong and to embrace the goodness of the light. They do so, Hetra in the lead. He wonders, with all the miracles he has seen recently, if the gift of flight will be given back to the Optera as well. Vrestin tells him that it will take a generation or so before that can happen. But she reminds him that Vortis and its light gods provide for all the creatures here, regardless of flight. They shall all live together again on the surface.

Prapillus, too, is pleased to have lived long enough to see the planet free again, but he is more stoic in expressing his feelings. He tells the Doctor that Vortis owes him a great deal for all his help. But the Doctor wants only one thing from Prapillus - his ring back. And so Prapillus presents it.

Ian joins Vrestin, watching the Optera hopping happily about. Hetra agrees with Vrestin that the light is good! Vrestin asks if Ian and his friends will ever return to Vortis. Ian thinks it unlikely, but has learned during his time with the Doctor that the future is never certain. As Prapillus takes Vrestin off to look at their communications device, Ian rejoins the Doctor. The old man is pleased that they've not only succeeded in defeating the evil Animus, but that they have done so unscathed. Ian reminds him about the loss of his old school tie. The Doctor feigns forgetfulness, and only laughs when Ian reminds him in detail of the tie's origin and value.

The Doctor moves off into the TARDIS, still chuckling to himself. Ian follows, beckoning Vicki and Barbara to join them. The gathered denizens of Vortis back away from the police box and watch as it disappears with its distinctive grinding engine noise.

Hlynia is dumbstruck at this, but Prapillus takes it in his stride. These people have many unknown powers. The songs of their great deeds will be sung forever in the temples of light. And whenever Pictos aligns with the Needle of the Kings, all Vortis shall remember the Earth people who came to help them in their time of need. And even as Vortis' delicate web of life returns to normal, the Menoptra must be on guard that its lushness does not conceal another lurking evil like the Animus.