Auntie, the leader of Earth colony Phoenix, goes through the motions of sending out a distress call, although she knows there’s nobody listening within range of the colony transmitters. She then opens a broadcast to the other colonists and informs them that sunspot activity has decreased, making it possible to punch a transit signal through the interference and send select individuals back home. For fifty years, the colonists have been working to improve their physical fitness so they can survive the return journey; to celebrate the latest return, Auntie opens up the teleport system so the colonists can travel between the habitat cells and share time with social partners. The colonists were once able to mingle freely with one another instead of living solitary lives, but those days are past. Auntie signs off, weary of the lies she’s been telling the colonists; her robot servitor, Butler, insists that her choices are all logical and that she is doing the best for the colony, but she finds it difficult to see things that way. Perhaps for this reason, she has not selected colonist 478, Bellip, to make the return.

Aboard the TARDIS, the Doctor is repairing the temporal drift circuits while Peri worries about Erimem, who has still not recovered from her traumatic experiences in Tibet. She asks the Doctor whether he regrets letting Erimem travel with them, but the Doctor assures her that he’s very happy with both his companions. Peri sets off to talk to Erimem, and finds her sitting in a room that seems to be a wide open field; however, to Peri’s distress, the disillusioned Erimem shows no wonder at her surroundings. Peri points out a road in the distance, and realises too late that she’s calling attention to Erimem’s injured eyes, which have not yet recovered from the effects of her possession. Erimem admits that she has been thinking of leaving the Doctor and Peri; she still considers them her friends, but has wearied of the danger and the things she’s learned about herself. Peri urges her friend not to give up on them yet, and though Erimem hasn’t decided, she does agree to return to the console room with Peri to find out where they’ll end up next.

On Phoenix colony, a young colonist named Laroq is practicing his rusty social skills, trying to figure out how to ask Bellip to share her social time with him. Eventually, he gives up and just opens a comms channel to speak to Bellip; however, her social skills are worse than his, and she doesn’t even activate the video link so they can see each other. They are obviously fond of each other, but the mere thought of spending time with another human being sends Bellip into a panic, and she thus turns down his request to visit her. She suggests that he spend time with his friend Vidler instead, but Laroq is growing tired of Vidler’s paranoid conspiracy theories. Bellip does agree to consider seeing him at the next social if she’s ready, but it doesn’t sound as if she will be.

Peri and Erimem return to the console room, where they find that the TARDIS has materialised in orbit near an apparently deserted space station. The Doctor hops aboard the station for a closer look, and materialises in the station’s operations centre. The life support is still active, but there seems to be nobody aboard, although dozens of monitor screens are trained on a small colony on the planet’s surface. Erimem finds a porthole, but the sudden sunrise nearly blinds her still-sensitive eyes. Peri tends to the recovering Erimem, and once her friend is better, they set off to explore, leaving the Doctor to examine the computers in the operations centre. The door seems to be jammed, and when Peri forces it open, an alarm goes off; however, nobody arrives to investigate before it switches itself off, and the Doctor concludes that the station really is deserted.

In fact, Auntie and Butler have heard the alarm, and Butler investigates and detects at least three active life forms aboard the station, Medusa. Auntie, fearing the worst, sends Butler to investigate, and orders him to be prepared to take offensive action. While waiting for him to report back, Auntie contacts Bellip, who is deeply distressed when Auntie tells her that, sadly, she isn’t strong enough to make the return journey; she’s been exceeding her exercise targets and eating full rations, and the disappointment is nearly too much for her to bear. Meanwhile, Laroq receives an unexpected visitor: Vidler, who wants to know whether Laroq has followed his advice and stopped eating the rations, which Vidler is convinced are drugged to keep the colonists docile. Laroq does indeed seem more alert, which is why it occurs to him that Vidler has ported in before the social is due to start; however, Vidler reveals that the teleports are always open, and that the other colonists are simply too timid to use them without permission. Laroq does agree with Vidler that they should have more freedom, but Vidler then reveals that he’s organising a mass protest of like-minded individuals, and the thought of five or six people being in the same room together gives Laroq a panic attack.

The Doctor calms Peri and convinces her to let Butler deal with the situation. Butler escorts them via teleport to an unoccupied habitat cell, explaining that the colony relies on the teleport for internal travel and that the space station is merely a docking facility for extra-solar visitors. He then takes the Doctor to meet Auntie, leaving Peri to cool her heels in the habitat cell. Auntie is suspicious at first, but accepts the Doctor’s claim that he and his friends are here by chance. She explains that the isolated colony was founded by pioneers who seeking independence from the rest of the Federation, but the terraforming process has taken longer than anticipated and the colony is on the verge of extinction. As a result, the colonists have become isolated and withdrawn, and have forgotten what it’s like to socialise with others. When Butler explains that one of the Doctor’s companions is lost, Auntie initiates a search for her aboard the colony ship. The Doctor wonders why she’s not searching the other habitat domes as well, but when Butler begins to answer, Auntie shushes him and sends him back to check on Peri.

Fortunately, Erimem has been teleported to Laroq’s habitat cell. Laroq panics at the thought of having three people all in the same living quarters, and Erimem is surprised when Vidler reveals that Laroq has never seen more than two people together at the same time. Vidler calms down the panic-stricken Laroq, and explains to Erimem that this colony is a failure. It’s located far from the major space lanes, and is full of meteorites, comets and other planetary debris, so nobody else ever comes here. Due to heavy sunspot activity, it’s almost impossible to punch a transit signal back to human space, and only the strongest and fittest can survive the journey. Thus, the colonists have isolated themselves in individual habitat cells to avoid distraction while devoting their every waking hour to their strict diet and exercise programme. Those who have built up enough physical strength are selected for the return journey, and leave the colony, never to be heard from again. However, the colonists have become accustomed to their isolation, and are unable to cope with crowds or open spaces.

Butler ports back to check on Peri and offer her some refreshments, but when she requests permission to go out and visit the rest of the colony, he reveals that there are in fact only sixteen colonists on the entire planet. Butler then ports back to Auntie’s, and once alone, Peri hears a woman crying in the other room. This is Bellip, who is surprised when Peri calls out to her and innocently answers Peri’s questions about the teleport system. When Peri dials up Bellip’s code and ports over to her room, Bellip suffers a crippling panic attack, and Peri learns to her surprise that Bellip has lived alone in this room for almost all her life without ever seeing another person face to face. Peri tells Bellip that she’s a traveller from another world where humans mingle in great numbers and often go outside, but Bellip nearly has another panic attack at the thought. Peri is disturbed by Bellip’s behaviour, and is convinced that she’s been brainwashed into accepting this limited lifestyle, especially when Bellip admits that she sometimes speaks to a boy named Laroq and might like to see him some day. Peri tries to convince Bellip that there’s more to life than just diet and exercise, but Bellip begins to panic again at the thought and, albeit apologetically, begs Peri to leave her alone. Peri reluctantly ports back out, convinced that Bellip is in need of help.

Vidler explains to Erimem that the colonists can only move between the cells by teleport and that such movement is monitored by the computer. Erimem insists that there must be another way out of the cells, and Vidler realises that it’s never even occurred to him to look for one. Laroq protests as Erimem and Vidler move his furniture about and tap the walls, but eventually Erimem finds a hidden panel. It appears to be locked at first, but eventually they realise that it was just jammed shut from disuse. Despite Laroq’s protests, Erimem and Vidler bash it partly open with Laroq’s exercise weights and force it the rest of the way. Laroq reluctantly accompanies Vidler and Erimem outside, and they find themselves in a curving corridor with many more doors like Laroq’s. Meanwhile, on the space station, General Makra’thon of the Khellian race is woken by his Queen, who has laid the last of her eggs and will soon die, her purpose served. Makra’thon must now wake the rest of his troops to protect the new brood -- and as the troops have been hibernating for some time, they will require a great deal of food...

Back in Auntie’s habitat, the search for Erimem has not turned up any results. The Doctor ports back to check on Peri, and once she is alone with Butler, Auntie confides in him that she’s worried about the Doctor’s unexpected arrival, which will complicate her own agenda. Meanwhile, Peri ports back into the unoccupied cell to find the Doctor looking for her, and she tells him about Bellip -- and about the eggs that she and Erimem saw on the space station. Like Erimem, the Doctor concludes that the habitat cells must have ordinary doorways, and he and Peri soon find this cell’s door hidden behind a hanging rug on the wall. They find their way out into the corridor, and its curvature convinces the Doctor that they are on the original colony ship; for some reason, the colonists are not living in the habitat domes. The next cell along is a door with Bellip’s teleport code on it, but when the Doctor and Peri enter, they find the room empty; Bellip must have been so frightened by Peri’s tales of the outside that she fled her room, too panicked to sit still. Peri decides to use the port to follow Bellip to her last destination, while the Doctor ports back up to the space station to take a look at the eggs for himself.

Makra’thon advances on the Doctor and Erimem, claiming to prefer fresh meat to frozen. Erimem tries to flee, but Makra’thon shoots her in the back, stunning her. He identifies himself as the General of the Khellian Royal Guard, and questions the furious Doctor, realising that he’s not one of the human colonists. The Doctor realises that the Khellians, a militaristic race of reptiles, have turned the colony below into a farm and the colonists into their food stock. Makra’thon insists that the stronger Khellians have the right to prey on weaker species, but he’s paying too much attention to the Doctor and not enough to Erimem, who is recovering quickly. Thus, when the Doctor loudly points out that the Khellians have the same physical weak spots as other humanoid species, Erimem hits Makra’thon in one of those weak points, and she and the Doctor flee before he can recover.

Peri follows Bellip to a small meeting hall, where she finds Bellip cowering under a desk and hyperventilating. Peri calms her down, but Bellip is ashamed of herself for suffering a panic attack even in such a small hall; perhaps this is why she’s never been selected for the return. Peri helps Bellip to walk out of the room, but Bellip can only take slow, shuffling steps. Peri compares this to walking with her grandmother, and learns that Bellip never even knew her own mother, let alone her grandmother; like the other colonists, she was raised by servitors like Butler. Nevertheless, Peri points out that the existence of this meeting hall proves that the colonists were once social creatures, and perhaps could be again.

Vidler and Laroq grow worried when Erimem fails to return, and the recall button also fails to retrieve her, indicating that she’s stepped off the dais. Vidler is still too frightened to follow her into the unknown, but still wishes to explore the rest of the colony ship; thus, he makes a note of the teleport’s settings so he can follow Erimem later if necessary, and then heads out into the corridor, with the reluctant Laroq tagging along. As they explore, they find only empty habitat cells, and realise that, between themselves, they know less than a dozen people altogether; there must be fewer people in the colony than they thought. Meanwhile, their presence in the corridors triggers alarms in Auntie’s cell, and when Butler runs a check on the other habitat cells, he finds that Vidler, Laroq, and Bellip are all unaccounted for. Horrified, Auntie orders Butler to send in the servitors for the missing children; Bellip must be retrieved unharmed, but the boys can be brought back alive or dead.

The Doctor has heard of the Khellians before; their queen produces thousands of eggs before dying, and the Royal Guard protects the eggs and the newly hatched brood while they are still vulnerable to attack. The station’s power plant has been turned into a nursery, and there are two Khellians standing guard on it now; fortunately, they were still hibernating when the Doctor and his companions first arrived. The fact that they’ve woken indicates that the Queen is nearly ready to die, which means that the eggs will hatch and the colony will be overrun. The Doctor thus returns to the operations centre, scans the colony ship and locates Peri, and then teleports down to warn Auntie of the danger, telling Erimem to hide and keep an eye on the Khellians. Meanwhile, the Queen summons Makra’thon, tells him that the time has come, and dies. The Khellians move her body into the centre of the room so that it will be the first thing the hatchlings see, and Makra’thon then leads his troops to the operations centre; they will need food to sustain them in the coming days, and Makra’thon intends to lead them on a hunt. Erimem watches from hiding as the Khellians board the teleport dais and beam down to the colony.

As Peri and Bellip shuffle through the corridor, they encounter Vidler and Laroq coming from the other direction. Bellip faints, partly from the shock of seeing four people together in the same space and partly from seeing Laroq face to face at last. She eventually recovers, and despite her nervousness, admits that she’s happy to see Laroq at last. Vidler compares Peri’s attitude to Erimem’s, but then must confess to the furious Peri that he let Erimem teleport into a potentially dangerous situation and didn’t follow her when she failed to return. Butler then ports in to capture them, but Peri and Vidler draw him off while Laroq and Bellip hide. However, as they flee, they run into a Khellian and are forced to retreat into a disused habitat cell. Peri orders Vidler to port out after Erimem while she hides in the cell; the Khellian assumes that they have both ported out, and once it’s gone, Peri doubles back to join Bellip and Laroq. In her absence, she learns that Butler has ported out as well, presumably to warn Auntie about the Khellian presence.

In fact, both Butler’s arrival and departure were due to the Doctor, who has teleported to Auntie’s habitat cell to demand answers. When he tells her where Peri is located, Auntie sends Butler out to collect her and the others. The Doctor then warns Auntie of the threat on her space station; the Khellians have been intercepting and eating the colonists whom Auntie has selected for the return, and soon they’ll be here to turn the entire colony into their farm. However, to the Doctor’s surprise, Auntie responds by calling back Butler and ordering him to restrain the Doctor. She loses her temper when the Doctor protests, insisting that she knows the price of survival better than he ever could -- and the Doctor realises that Auntie has known about the Khellians all along and has deliberately been sending her colonists to the station to serve them up as food. Auntie, enraged, orders Butler to kill the Doctor if he speaks again.

Fortunately, the Khellian hatchlings ignore Erimem and Vidler, moving past them to feed on the corpse of their mother. Repulsed, Erimem and Vidler retreat. Meanwhile, down in the colony, Auntie orders Butler to kill the Doctor, but he hesitates, unsure whether this is a flawed order or not. The Doctor has now realised that most of the colonists were left frozen in cryogenic stasis at planetfall, and concludes that Auntie struck a deal with the Khellians, buying her own survival by sending them the bodies of the frozen colonists as food. He boards the teleport dais, intending to return to the station and put a stop to this, but Auntie orders Butler to shoot the Doctor if he reaches for the controls. Before Butler can react, however, the Doctor is ported out; Erimem and Vidler have returned to the operations centre and activated the recall circuit.

The Doctor shuts down the entire teleport system; now Auntie will be unable to send Butler after him and the Khellian Royal Guard will be unable to return to the station. He then contacts Peri and orders her to get Laroq and Bellip out of the colony ship; they may be unable to cope with the outside, but inside, they’ll fall victim to the Khellians. The Doctor leaves Erimem and Vidler to guide Peri through the colony ship to safety, and instructs them to remove the radiation shields from the power generators while the Khellian hatchlings are still vulnerable; the Doctor is reluctant to slaughter the hatchlings, but sees no other way to save the colony. He returns to the colony in the TARDIS, hoping to talk sense into Auntie, and as Erimem watches the TARDIS vanish before her eyes, she realises for the first time what it will feel like when her travels with the Doctor and Peri come to an end. Nevertheless, she pulls herself together and starts to guide Peri to the colony ship’s airlock. Vidler finds that he can’t raise the radiation shields remotely from the operations centre; he’ll have to don a radiation suit and raise them manually, risking attack from the hatchlings. He shows Erimem how to reactivate the teleport system for just a minute so he can beam out to safety once he’s finished.

The Doctor returns to Auntie’s habitat cell, where she insists that he reactivate the teleport system so she can do what’s best for the colony. The sunspot activity set back the terraforming process by decades, but the colony ship’s engines are useless and the transit link back to the Federation has broken down, leaving the colonists with nowhere to run when the Khellians arrived. Outnumbered, Auntie struck a deal with the Khellians, who agreed to send her family members to safety in exchange for food; she has been buying time for the terraforming process to complete itself, intending to wake the rest of the colonists and fight the Khellians once the planet’s surface is capable of supporting them. However, the Doctor now reveals that the Khellians have betrayed her; according to the computer records he saw when he first arrived, there have been no deep-space transmats from the station. The Khellians prefer fresh meat to frozen, and Auntie’s family members have all been eaten alive.

Devastated, Auntie admits that some part of her always suspected the truth, which is why she never selected her grand-daughter, Bellip, for the return. Furious, she orders the computer to switch off the colony ship’s life support and drain the corridors of oxygen. The Doctor volunteers to act as bait and distract the Khellians so they don’t notice the drop in atmosphere until it’s too late. On the Doctor’s instructions, Butler contacts Makra’thon and tells him that the Doctor is on board, and Makra’thon eagerly sets off to hunt down the man who’d made a fool of him. He finds the Doctor more quickly than the Doctor had hoped, however, and Butler is forced to intervene, ushering the Doctor into an empty habitat cell and helping him to escape down a service shaft used by the servitors. When Makra’thon learns that his prey has escaped, he shoots Butler, destroying him. The Doctor returns to Auntie’s habitat, where he learns, to his horror, that she’s taken more drastic action; the atmospheric drain was taking too long for her to be certain it would kill the Khellians, and she’s thus primed the colony ship’s self-destruct countdown and raided the armoury. The Doctor and Auntie retreat into the TARDIS and dematerialise as the countdown approaches zero.

Peri is guiding Bellip and Laroq through the corridors with difficulty, as Bellip can move little faster than a shuffle through unfamiliar territory. With Erimem’s help, they reach the airlock, but Bellip simply freezes, unable to move, when Peri opens the outer door and insists that they must cross 100 yards of open space to the door of the nearest habitat dome. Even Laroq needs help to get that far, and Peri thus gives in and decides to help them across one at a time. With Peri’s help, Laroq crosses the open space to the habitat dome, and despite his fear he finds the experience exhilarating; however, when they arrive at the habitat dome, Erimem contacts Peri and warns her that the station’s systems have detected that the colony ship is about to self-destruct. Peri rushes back to the ship, coaxes the panic-stricken Bellip out of the airlock, and urges her to close her eyes and pretend that she’s exercising on her treadmill, back in the habitat cell. With great difficulty, she manages to get Bellip walking as fast as she can, and they reach the habitat dome and seal themselves inside just as the colony ship explodes. Laroq and Bellip’s cells have gone up with it; like it or not, their new lives begin today.

The Doctor and Auntie watch the explosion from the safety of the TARDIS, and the Doctor, upset, sets course for Medusa to pick up Erimem and Vidler. However, things have not gone smoothly aboard the station; although Vidler successfully raised the radiation shields, when Erimem reactivated the teleport system to beam him out, Makra’thon seized the slim window of opportunity to port himself to safety. When the Doctor emerges from the TARDIS, Makra’thon is holding Erimem hostage, enraged that his loyal troops have been slaughtered by creatures he considers no better than animals. As far as he’s concerned, the Khellians have a noble civilisation worth preserving, while humans just spread across the galaxy like a plague, bringing their narrow-minded morality and hypocrisy with them. He is even more enraged when he learns that the hatchlings are now dying from radiation poisoning, but he is still disoriented from the lack of air on the colony ship, and before he can take his revenge, Erimem wriggles out of his grasp -- and Auntie attacks him with a bomb she took from the ship’s munitions locker, blowing him to bits.