Following the Black Guardian's instructions, Turlough locates a particular roundel in the TARDIS, and operates the controls which will enable him to remove a vital component from the heart of the ship. Tegan finds him apparently talking to himself next to the open roundel, but he's adept at turning her arguments against her; why should he wish to damage the TARDIS now that he travels in it? Tegan, still suspicious, takes him to his new room, which was once Adric's, and leaves him to clear out all the "junk". As soon as she's gone, Turlough heads for the console room, where he finds that his crystal link to the Black Guardian has repaired itself, and sets about tearing the space-time elements out of the TARDIS console.

Nyssa is trying to keep her biotechnical skills in practice, but is having trouble, as the last time she synthesized an enzyme Adric helped her with the calculations. Tegan goes to fetch Adric's notes and is surprised to find Turlough missing -- and when she tries to return to her own room she finds the walls disintegrating. She calls the Doctor, who rushes to the console room to conduct repairs. Turlough, having failed to completely free the space-time element, shoves it back into place and adopts a posture of innocence moments before the Doctor and Tegan arrive. The Doctor tunes the scanner into the TARDIS interior, and sees that Nyssa has been trapped in her room by the instability. The outside Universe is breaking through the dissolving TARDIS, and a door with a half-skull motif appears in the wall behind Nyssa. The Doctor urges her to pass through the door before the instability consumes her. The screen blazes with light, and when it clears Nyssa is nowhere to be seen.

The Doctor rushes to Nyssa's room just in time to stop the new door from closing, explaining to Tegan and Turlough that a safety cut-out enabled the TARDIS to lock onto a passing spaceship and use it as an anchor. The Doctor finds Nyssa's scorched notebook in the corridor outside and sets off in search of her. Moments later, Tegan hears a howl of agony in the distance and rushes off to investigate. The Black Guardian orders Turlough to follow her, and as he leaves the TARDIS, the door slams shut behind him and vanishes into thin air. He and Tegan have no choice but to go on.

The Doctor finds Nyssa hiding in the corridors of the ship, which appears to be a deserted passenger liner -- although the corridors seem to echo with eerie, distant cries and howls. Elsewhere, another spaceship docks with the liner, and two raiders blast a hole in the hull. As soon as they enter, the hole is sealed up to prevent the air from escaping. Once inside, however, Kari and Olvir find that the liner doesn't match the briefing they were given -- despite the breathable atmosphere it's apparently deserted and the bridge is fully automated. The Doctor and Nyssa take a wrong turning on the way to the TARDIS and end up on the bridge, where Kari and Olvir hold them at gunpoint and demand answers. But the raiders' ship suddenly cuts out on them and flees, abandoning them to their fate. The sounds from the engines have changed pitch, indicating that the liner is preparing to dock with something, and Kari and Olvir have no choice but to ask the Doctor for help. He agrees, on condition that they put away their guns.

Tegan and Turlough encounter a robot drone which appears to ignore them. When they hear a plaintive female voice crying out from behind a sealed door, Tegan concludes that it's Nyssa and sends the reluctant Turlough to find some means of prying open the door. Turlough happens to find the entrance to the TARDIS shimmering back into existence as the engine sounds start to change. He considers returning, but instead goes back for Tegan, who has managed to inch the door partway open -- and is now fighting off the diseased hands which are clawing through the gap at her. Turlough pries her free of their grasp, and they retreat as the door slides shut. Tegan and Turlough head back for the TARDIS, unaware that once again its door is fading from existence.

The Doctor stops Kari from shooting the shuffling "Lazars", most of whom are too ill even to react to their presence. He also refuses to abandon the panic-stricken Olvir, who knows more than they do about Terminus. He takes Nyssa and Kari back to the bridge to find more information about the station. Kari explains that the raiders chose this ship because it had just made several stops in a rich sector; they therefore assumed it had a cargo worth stealing. While the Doctor studies the ship's star charts, Nyssa -- shaking off a moment of dizziness -- searches for more data cubes and finds Olvir hiding from Kari, ashamed of his moment of cowardice. She convinces him to rejoin the others, and he informs them that his sister died of the Lazar's Disease on Terminus. The company which runs Terminus is interested only in profits, and Olvir has never met anyone who claimed to have been cured there; of course, he's never met anyone who would admit to suffering from the loathsome disease in the first place. The Doctor's curiosity is piqued by the location of Terminus -- it may be a coincidence, but the station is at the exact centre of the known Universe.

Tegan and Turlough find that the Lazars walking over the floor panel have wedged it into place. As they crawl through the ducting, searching for another way out, they are nearly exposed to lethal levels of radiation from the poorly shielded panels. Tegan tries climbing to a higher level, and Turlough follows, the Black Guardian's warnings echoing in his head; there are penalties for failure, just as there are rewards for success... At the top of the ladder, they find another sealed panel, but before they can retreat they see a man walking through the ship's corridors, having completed a rudimentary inspection. The man, Eirak -- who is dressed in a uniform like a cross between armour and radiation shielding -- orders the drones to begin sterilisation procedures. Toxic gas floods through the ducting, but fortunately it is vented down the shaft which Tegan and Turlough just climbed, and they survive on the outskirts of the cloud.

The Doctor and Nyssa find and print out a layout of the ship, and they and the raiders split into two parties to cover the ground more quickly and find the TARDIS before the automated sterilisation procedures begin. The Doctor and Kari set off in one direction while Nyssa and Olvir set off in the other, but Nyssa, who still hasn't recovered from her experience in the disintegrating TARDIS, must pause to rest. Feeling flushed, she takes off her skirt while Olvir tries to contact Kari and fails due to the radiation leakage on the ship. As a robot drone approaches them, Olvir starts to pull Nyssa to safety... and then snatches his hand back as if burned, realizing from Nyssa's sallow complexion and the dark circles under her eyes that she's fallen victim to the contagion surrounding them. Nyssa has been infected with the Lazar's Disease. The drone snaps its arm around Nyssa and drags her off as Olvir turns away, ashamed but unwilling to go near her as she pleads for help.

Once off the ship the Lazars fall into the desultory hands of the Vanir, the "baggage handlers" of Terminus. One of the Vanir, Bor, finds that the readings he is monitoring are increasing, and deliberately enters the high-radiation Forbidden Zone to investigate. Despite his friend Valgard's protests, the bureaucratic Eirak refuses to send anyone in after him and seems more concerned with the loss of Bor's armour. Sigurd advises Valgard not to anger Eirak, who controls the distribution of Hydromel, the drug which keeps them alive. Nyssa is brought to the Vanir, and is appalled by their lacklustre attitude towards the Lazars. She was expecting to find a cure for her condition, but there are no doctors on Terminus; there are only the Vanir, who hand the Lazars over to the Garm, a creature which lives in the Forbidden Zone. What happens after that is anyone's guess.

Sigurd enters the ship to collect the latest shipment of Hydromel, which the drones neglected to bring out along with the straggling Lazars. Nyssa tries to escape, and as Valgard isn't taking her seriously or paying close attention, she is able to overpower him. She tries to flee back into the ship, but runs into the returning Sigurd. As Sigurd and the embarrassed Valgard drag Nyssa off into Terminus, Olvir, having overcome his revulsion, emerges from the ship and follows them. In the depths of Terminus, Nyssa, her pleas ignored, is given sackcloth to wear and is flung into a crowded, cramped room with the other milling, hopeless Lazars. Eirak, meanwhile, orders Valgard to tell the Garm to keep an eye out for Bor and to bring back his body. Valgard does so, unnerved by the silent, looming dog-like creature and its burning red eyes.

Having failed to find either the TARDIS or the singed notebook which was marking its position, the Doctor and Kari stumble across Nyssa's abandoned skirt and find blood on the floor. Tegan and Turlough, in the ducting nearby, hear the Doctor and Kari's voices and try to call out, but just as they're starting to catch the Doctor's attention, the Doctor and Kari are forced to retreat from a cloud of sterilising gas. Stranded in the ducting, Turlough angrily hammers at the grating before him and is just as surprised as Tegan when it snaps free.

Kari, recovering but unable to get a clear shot at Valgard, fires at a set of metal panels instead. The beam reflects from the panels and strikes Valgard on the helmet, but Kari's power pack has run out, and she and the Doctor flee into the Forbidden Zone while they have the chance. Valgard recovers and stumbles off to tell his story to Eirak, who has just found that the amount of Hydromel in the latest shipment has been reduced. Like Valgard, Eirak concludes that the strangers must be Company spies. They must be caught and forced to explain why the Hydromel shipment has been reduced, but Eirak is unwilling to follow them into the Forbidden Zone. When Valgard challenges Eirak's authority, Eirak in turn challenges him to fetch back the spies; if he succeeds, Eirak claims he will step down as leader of the Vanir.

Nyssa tries to get the Vanir to respond to her, but fails, much to the cynical amusement of a Lazar named Inga who feels certain they've been abandoned here to die. Olvir, meanwhile, disguises himself as a Vanir, but is nearly caught by Sigurd in the equipment room. Sigurd fetches Eirak, believing he's found another spy, but when they arrive they find only a stack of uniforms in the vague shape of a man and Sigurd is made to look a fool. Olvir reaches the border of the Forbidden Zone, where Valgard, hesitating almost superstitiously, thinks he's a Vanir come to goad him on and angrily enters the Zone.

Tegan and Turlough finally emerge from the ducting. The sterilisation procedures are complete and the drones take no interest in them as they aren't Lazars. When they pause for a rest, the preoccupied Turlough asks Tegan if she could ever kill someone in cold blood; she finds it an odd topic of conversation, and doesn't answer. They eventually find the bridge but aren't sure what to make of the maps which indicate they are at the centre of the Universe. Unable to find any way to reopen the door to the TARDIS, Turlough tells Tegan to wait for a moment while he returns to the corridors. Once out of earshot, he contacts the Black Guardian via his crystal cube. The cube heats in his hand, punishing him for his failure, but when he insists that he has a plan the Black Guardian tells him to look beneath his feet for the solution. Tegan comes to find him, and he pockets the cube just in time and tells her that he's remembered seeing an emergency bypass switch in the floor beneath the TARDIS.

Kari is unable to contact Olvir due to the radiation leakage interfering with her communicator, but the Doctor is able to use the communicator to trace the source of the leak. He and Kari meet the delirious Bor, who has been badly injured by trying to pull down power lines which he thought were control lines for Terminus' engines. He is now trying to build a wall of scrap metal in front of the damaged engine as a radiation shield; one of the engines exploded a long time ago, and if this one goes nothing in the Universe will be safe. Valgard arrives to capture the Doctor and Kari, but decides to beat some respect into them first. The Doctor overpowers the radiation-sick Valgard, knocking him into Bor's makeshift wall; before Bor can do anything about it, the Garm arrives to collect him. At that moment a sonic signal summons the Garm back to the border of the Forbidden Zone, and since he has been given no instructions regarding the Doctor and Kari, he leaves them. The Doctor and Kari leave Valgard to recover and follow the engine's power lines, as the Doctor suspects Bor's story may turn out to be more significant than it appears.

The Vanir move the strongest of the Lazars first, and that means Nyssa, although she insists there are others in more desperate need of the cure. They chain her up near the Forbidden Zone to wait for the Garm, who arrives carrying Bor. As Sigurd and the others take Bor back to Eirak, one Vanir remains and reveals himself to be Olvir in disguise. He is unable to free Nyssa, and when he tries to shoot the approaching Garm, the creature simply ignores the beam from his gun. Nyssa is too weak to resist as the Garm picks her up and carries her into the Forbidden Zone, and Olvir follows them to the engine area. The Garm chains Nyssa up next to the leaking engine, but before Olvir can rescue her Valgard attacks him. Meanwhile, Bor rambles deliriously, telling Sigurd that the dead pilot of Terminus is about to fire up the engines again and they won't take it. Eirak is angry to see that Bor's damaged armour is unsalvageable, and refuses to spare any Hydromel for him, since he's dying in any case.

The Doctor and Kari spot a large control lever moving towards the closed position, but are unable to hold it back. They would need the strength of a giant to do so. Fortunately, they know where one can be found.

Olvir eventually overpowers Valgard, only to find that the Garm has taken Nyssa away. Valgard has recognized Olvir's combat moves and realizes that he isn't a company spy at all, but a raider trained by Colonel Pereira; Valgard also used to raid under Pereira as well, until Pereira turned him in for the reward. All of the Vanir are criminals, sold to Terminus Inc. as slaves and bound to serve by their addiction to Hydromel. Olvir's gun has fallen into a high-radiation zone near the engines, and he is forced to leave it behind when he sets off in search of Nyssa. But Valgard, who wasn't as weak as he was pretending, uses his staff to pull the gun out of the irradiated zone. Meanwhile, the Garm places Nyssa in an empty metal room, where she awakens to find that she has been cured of the disease.

Tegan and Turlough manage to activate the emergency bypass, but as the door to the TARDIS begins to materialize, the automated tannoy announces that the liner is about to depart. Realizing that the Doctor is about to be left behind, Tegan rushes to the bridge despite Turlough's insistence that there's nothing they can do. Moments later, the TARDIS door appears, and Turlough goes through without waiting for her. Tegan is unable to get the controls to respond to her; frustrated, she thumps them with her fist, and for some reason the automatic launch procedure halts. She returns to the TARDIS door, only to find that it has vanished again, and Turlough has gone with it. Turlough reaches the TARDIS control room and contacts the Black Guardian, claiming that the Doctor is trapped, powerless and probably dead already. But the Black Guardian isn't convinced; Turlough must be punished for his failure. The crystal cube blazes with light...

Olvir locates the Garm and demands to know what he's done to Nyssa. To his surprise, the Garm speaks to him, assuring him that Nyssa has been cured. The Garm leads Olvir to the decontamination chamber but must then leave in response to a signal sent from the control box. Inside, Olvir finds that Nyssa has been cured, but she's appalled by the indifferent nature of the "treatment". The disease was driven from her by exposure to a massive dose of radiation with no controls, checks or balances. With proper controls, Terminus could become a real hospital; as it is now, those who leave cured of the Lazar disease may end up suffering from other, radiation-induced sicknesses for the rest of their lives.

The Doctor and Kari have summoned the Garm, and they are surprised when he speaks to them, agreeing to follow without their needing to use the signal box to lure him onwards. Terminus quakes as the computer prepares to jettison the engine, but the Garm exerts all his strength in an agonising effort to return the control lever to its starting position. The Doctor tears free the control wires, ending the immediate threat, and in exchange he smashes the signal box, ending the Garm's servitude to Terminus Inc. While the Garm relaxes for the first time in years, the Doctor and Kari set off to pull down the control cables between the bridge and engines to ensure that the ejection procedure will never be activated again. Before they can do so, however, Valgard appears and holds them at gunpoint, but as he tries to march them back to the holding area, Nyssa and Olvir surprise him and overpower him. Nyssa tells the Doctor about her experience, and explains to the sceptical Valgard that if the Vanir work together with the Garm, Terminus could become a proper hospital. Terminus Inc. only controls Terminus because it controls the Vanir's supply of Hydromel, but what if the Vanir could manufacture their own Hydromel on Terminus?

Bor's condition is steadily worsening, and he can no longer remember what he found in the Forbidden Zone. Sigurd tries to steal some Hydromel for him, but the box is sealed and only Eirak holds the key. Or so it seems until Valgard arrives, accompanied by the Doctor and Nyssa. The Doctor breaks open the Hydromel case, and Nyssa analyses a sample and confirms that she can manufacture her own -- and perhaps even improve the formula. Sigurd takes some Hydromel for Bor, and when Eirak returns, demanding to know where the Garm's signal box has gone, he finds Valgard, Sigurd and the recovering Bor waiting to remind him of his promise to step down should Valgard return with the intruders. There are going to be some changes around Terminus.

Kari and Olvir destroy the control cables, ensuring that Terminus will no longer pose a threat to the Universe. Now all they have to do is work out how to get home. The Doctor, Valgard and Nyssa return to the liner, where the Doctor is irritated to see an exhausted Tegan wandering the corridors, looking for him. While Nyssa speaks with Tegan, the Doctor advises Valgard to speak with the Garm and win its confidence. Terminus Inc. may threaten to send retaliation, but the stigma borne by the Lazar disease should keep them safe until they can get out word of the changes. At that point, Nyssa drops a bombshell; following her traumatic experience, she's decided to remain on Terminus and use the skills she learned on Traken to turn it into a true hospital. It will be a hard life, but she's willing to make the effort. She bids a tearful goodbye to the Doctor and Tegan.