After six years’ training, crewman Donald Philpott is chosen to be the first man in years to step onto the planet’s surface. But what should have been triumph becomes tragedy, as within seconds of his capsule door opening, Philpott collapses screaming in terror...

The TARDIS materializes in an abandoned cinema in what seems to be 1950s London -- but Nyssa knows that cities on Earth aren’t built inside stone caverns to protect them from the frozen atmosphere, and that Earth isn’t located in deep space, far closer than comfortable to the Cherrybowl Nebula. The Doctor seems disturbed, but reluctantly gives in to Nyssa’s desire to explore, telling her to meet back at the cinema in half an hour. Nyssa soon finds trouble; young Yvonne Hartley has been helping her father to trap pests, but while setting a “cheeser” to lure one such pest out from hiding, her father has inadvertently brought a stack of timbers crashing down on himself. Nyssa helps Yvonne to dig her dad out from under the timbers, but when she examines him she can’t detect any breathing or pulse. As she tries to break the bad news to Yvonne, however, Hartley sits up, shaken but alive. He and Yvonne thank Nyssa for her help, but then the curfew sirens start to wail and they insist that she return home with them. She tries to explain that she must return for her friend, but they won’t take no for an answer.

The Doctor, exploring elsewhere, meets a far less savoury character -- Thomas Dodd, “Purveyor and Fitter of Necessary Body Parts.” Dodd -- who deals only in proper body parts, not the new artificial organs created by the Committee -- notes the Doctor’s excellent health and is quite interested to learn that he has no identity papers. The Doctor turns down Dodd’s offer of a drink, but does ask about the city... which, as he’d suspected, is the last inhabited city on the planet Mondas. The Doctor decides to return to the cinema, but he and Dodd are confronted by a policeman who, like his horse, has been augmented with cybernetic parts. However, the level of augmentation isn’t as advanced as the Doctor had feared, and by lighting a firecracker under the horse’s nose, he frightens the animal long enough for himself and Dodd to flee. The policeman transmits a report to his superiors, warning them that there are intruders in the restricted area of the city.

Yvonne and Mr Hartley usher Nyssa into their apartment, where the sullen young Frank is waiting with Sisterman Constant, a public sector nurse of sorts who has come to check on her charges. Yvonne and her father both claim to be feeling well, although this clearly isn’t true; Yvonne requires pills to survive her frequent coughing fits, and Hartley pulled his arm earlier when the timber stack fell on him. Constant demands to know who Nyssa is, but Nyssa stands up to her questioning and sends her firmly on her way. Frank tries to volunteer for service, but is disappointed when Constant, like his father, claims that he’s too young. Constant takes her leave of the Hartleys -- but, still upset by Nyssa’s insolence, she phones in the presence of a selectee for the work crews, and of a stranger without identity papers.

As Frank works on his logic and cybernetics homework, Nyssa learns that the Hartleys’ pet bird is partly mechanical. It’s not the only one; now that Constant has gone, Hartley confesses that his own chest box is playing up, and asks Nyssa to take a look at it. As she does so, she hears a commotion outside as one of the Hartleys’ neighbours, Mrs Ginsberg, is caught out after curfew. The Hartleys refuse to answer the door, turning up the television to drown out her pathetic cries for help as the police drag her off. Nyssa is quite disturbed, but the Hartleys are more disturbed by the sound of police patrols on the street, an entire convoy on manoeuvres in the dark...

The Doctor and Dodd see the same trucks trundling past, and the Doctor realizes that the police are keeping people away from this area because something illicit is being done under cover of darkness. He demands that Dodd help him find out what’s going on, and Dodd reluctantly leads the Doctor to the bell tower of the long-abandoned Church of Former-Day Souls. There, they make their way past the giant cogwheels to the tower, where they see the whole city laid out before them -- and, more to the point, the graveyard, where augmented policemen are digging up the corpses. Even Dodd is appalled by this. The Doctor and Dodd are then spotted and arrested by a policeman, who is killed in the ensuing struggle. The Doctor winds the bell ropes around the pendulum and cogwheels, and the bells begin to toll out a wake-up call to the people of the city. The Doctor may be unable to prevent what’s happening here, but he can let the people see for themselves, and give them a chance to change their own future.

Nyssa has repaired Hartley’s chest unit, and he and Frank do the dishes while Nyssa and Yvonne hang tinsel and baubles on the tree. Yvonne is surprised to find her old ‘mat, “Matty”, hiding in the decorations box, and explains to Nyssa that her father catches the wild ones. The happy family atmosphere reminds Nyssa of her own long-lost home, but this family has its problems, as Frank is jealous of the attention his sister gets and is angry that his father won’t let him join the work crews. Frank believes that the crewmen will soon reach the surface, and he wants to be a part of the heroic campaign -- but his father knows of no one who’s ever returned. Furious, Frank storms out, accusing his father of wasting their rations on strangers. Nyssa apologises for the trouble she’s caused and insists upon replacing the food they’d given her, but at that moment the police arrive in response to Constant’s report. Hartley delays them while Yvonne sneaks Nyssa out the back, giving her “Matty” as thanks. When the police threaten Hartley, Frank directs them to the back room, but fortunately Nyssa has gone. As the Hartleys wait to see what the police will do with them, they hear the sound of the church bell, tolling as if in warning.

Nyssa refuses to stand by and let the good people of Mondas suffer, but the Doctor insists that they can’t risk interfering to such extent. The Cybermen are infamous throughout the galaxy, and their absence would change the whole of history. The argument escalates until Nyssa snaps and brings up the death of Adric, and the Doctor acknowledges that they never mourned him properly. Nyssa calms down and apologizes, and the Doctor, understanding her frustration, agrees to let her take some food to the Hartleys. However, Nyssa casually mentions that Yvonne gave her a pet, and the Doctor loses his temper when he realizes that there’s a Cybermat on board his TARDIS. Attracted by the power from the console, the Cybermat burrows into the energy conduits and burns itself out feeding, seriously damaging the TARDIS in the process. The Doctor, upset, allows the contrite Nyssa to begin repairs while he goes out for a walk to cool his temper.

Though the people are furious to see the police digging up the graveyard, the police stop the church bells and use force to disperse the angry crowd. Satisfied, Sisterman Constant returns to the Committee palace, where Doctorman Christine Allan is drowning her sorrows in wine. She can’t get the work crews’ processing to stabilize, and the Committee won’t give her the chance to slow down and rethink her approach. She’s amused to hear of the riot, as she never approved of using the bodies from the graveyard as resources. She’s also quite intrigued by Constant’s claim to have encountered a stranger, and decides to send surveillance Cybermats out to see for herself. She is then summoned before the Committee, who order her to increase the rate of processing; more workers are needed urgently. As the rigours of full processing usually kill the subjects, Allan refuses to go ahead until the Committee explains why this is necessary...

Morning has broken. Frank, sent out by his father to look for Nyssa, finds the Doctor cooling his heels outside the TARDIS. The Doctor sends Frank in to speak to her while he deals with business elsewhere, and though astonished by the TARDIS interior, Frank pulls himself together and tells Nyssa that Yvonne got her call-up papers for the work crews. The whole apartment turned out, giving her a hero’s send-off which Frank feels should have been his. Yvonne went willingly, believing it was her duty, but their father fears that he’ll never see his daughter again. Nyssa is just as upset, but for different reasons; Frank admits that the surface work crews undergo some sort of augmentation, and Nyssa knows just what this means for Yvonne. Upon learning that “Matty” was responsible for damaging the TARDIS, Frank explains that the ‘mats are often attracted to sources of energy, which is how his dad traps them with his “cheeser”. The explanation comes too late, however, as Nyssa hears a chirruping from outside and a swarm of Cybermats covers the outside of the TARDIS, trying to get to the power inside...

Constant finds Allan drinking heavily, and chastises her until Allan reveals that the Committee has decided that the city wastes too many resources -- and that it will soon begin processing every citizen. The Sisterman is horrified, and, pleased to have cracked her smug shell, Allan sets off to check out the intriguing Doctor for herself. On her way out, she passes the new “recruits” as they line up for processing. Yvonne Hartley is in line, hoping that she’ll have the chance to show off her new uniform to her father before she’s sent up to the surface. She doesn’t understand the truth yet -- Constant chose her because she’s gravely ill and was likely to die within a few months anyway. And the “processing” involves far more than a new uniform. Yvonne begins to feel fear as the processing conveyer carries her forward, and as she sees what the other recruits have become, but it’s far too late...

The Committee is facing several problems. Although Doctorman Allan now understands the threat that faces Mondas, her emotional reactions remain a dangerous weakness. The riots in the city have cost the lives of two policemen, and according to Commander Zheng of the surface crews, Mondas is entering the region of unstable space sooner than anticipated. Since Allen has left the palace, the Committee makes its own decision, delays the emergency procedures and summons Zheng back to the city to restore order and ensure the threat is dealt with efficiently.

The Doctor flags down a police transport, spills its cargo of bones over the street in order to attract a crowd, and steals the transport and flees before the augmented policeman can stop him. He then returns to Dodd’s, where he finds Mr. Hartley trying to sell his body parts in advance in order to provide for Frank. Dodd turns down Hartley’s request for credit and sends him on his way while he speaks with the Doctor. The Doctor asks Dodd for help to break into the Committee palace, hoping to find out how far the Cyber-conversion process has gone. Dodd refuses to try anything so risky, however, and instead he tricks the Doctor and locks him in the cold storage units. He’s always in need of fresh supplies, and the Doctor is in perfect health -- or will be until he freezes to death.

Since Frank didn’t close the outer doors on his way into the TARDIS, the Cybermat pour into the gap between inner and outer doors and begin to force the inner doors open through sheer weight of numbers. Thinking quickly, Nyssa re-routes the power from the energy conduit to the outer shell, effectively turning the TARDIS into a giant “cheeser” and frying the entire swarm. She and Frank then take food back to the apartment, where Frank’s morose father is trying to cheer himself up by decorating the family tree. As he does so, a disturbing news item appears on the television; the Committee has chosen to publicize the new “suits” which they claim are used to protect workers against the extreme conditions on the planet’s surface. The horrified Nyssa realizes that the story is being spun to hide the truth; the figures on screen aren’t men and women in uniform, but fully processed Cybermen. The broadcast is then interrupted by a city-wide blackout, and she and the Hartleys hear distant explosions to the north...

Allan finds her ward in chaos; one confused recruit has already wandered off on its own, and the others are demanding to know what their purpose is. When Constant informs Allan that the Committee has fallen silent, the recruits -- already programmed to protect the Committee at all costs -- demand an explanation. When Allan fails to provide one, the recruits conclude that she and Constant are withholding information and thus pose a threat to the Committee. Allan manages to explain that the Committee will die if she doesn’t restart the palace generators, but the recruits don’t trust her and send an escort to keep an eye on her.

Outside, the temperature is falling, and as the city quakes, rubble from the roof falls on the Doctor’s guard, enabling him and Dodd to flee and hide in the palace. The Doctor knows what’s happening; Mondas is entering the turbulent zone of space known as the Cherrybowl Nebula, and the quakes are only going to get worse. As he scrapes gold leaf from the palace statues, he tries to learn more about the Committee from Dodd, but Dodd cares little for city politics and is more interested in finding the palace’s old wine cellars. The rogue recruit wanders past, looking for someone who can explain its purpose, and remembering only that it wanted to show its uniform to Father. Dodd is horrified by the sight of the creature, a thing made entirely of spare parts -- and is even more shocked when the Doctor finds a chamber containing racks of similar body suits, enough to process the entire population of the city into Cybermen.

Dodd flees, refusing to have any part of this horror, but the Doctor lags behind when he sees a Cyberman forcing Doctorman Allen along the corridor. The Doctor flings gold leaf into its chest unit, but this has no effect; nevertheless, the recruit realizes that it’s under attack and nearly kills the Doctor. Allan saves his life by operating the recruit’s abort switch, but is sickened by the death; however, the Doctor doubts that the recruit was truly alive anyway. The temperature inside the palace is dropping rapidly, however, and he and Allan must save the argument for later and get the generators going again at once.

Hartley is surprised to see snow outside, the first weather the sealed city has seen in a long time. Nyssa and Frank go out looking for the Doctor, but there’s no sign of him at the TARDIS, and it’s getting darker and colder by the minute. Convinced that the Doctor will be at the centre of things as always, Nyssa and Frank return to the apartment for fresh torch batteries, planning to try at the palace. Moments after their arrival, however, the rogue recruit from the palace arrives and batters down the door of the apartment, apparently half-crazed. It then stands in the middle of the apartment, seemingly confused and distressed -- and all present are horrified when Hartley tries to find out what the creature wants, and it calls him “Dad”...

Constant complains of hunger and a need to answer a call of nature. The recruits point out that her physical weaknesses could be eliminated by full Cyber-processing, but she informs them that this is only for those selected by the Committee. They conclude that she must know what their purpose is, and break her shoulder while trying to force the truth from her. Commander Zheng then arrives, and to Constant’s horror, Zheng guns down the recruit which was assaulting her. The rest of the staff have fled, demonstrating their unreliability when compared to Cybermen. He sends the confused recruits back to the processing chambers for reprogramming... and since Constant’s shoulder is broken, he sends her as well, despite her protests.

The Doctor is amazed by the size of the generators, and realizes that something other than the city is making vast demands on their power -- or was, before the energy fluctuations from the Cherrybowl Nebula blew out their fail-safe breakers. The Doctor wants to help for the sake of the people in the city, but Allan’s sole concern is getting the Committee operating again. The Doctor, realizing the truth, pushes past Allan and discovers that the Committee is in fact a cybernetic gestalt, twenty great minds joined together, their bodies withering in neglect as their minds commune electronically to provide clear and logical solutions to Mondas’s problems. It is the Committee which has been devouring the city’s resources, which is why a civilization with technology millennia ahead of Earth’s is stuck in the cultural equivalent of the 1950s.

The Committee is still functioning at bare minimum, and Zheng arrives and reports to them as the Doctor and Allen work to repair the generators. The Doctor listens to Zheng’s report, and learns the truth; the Cybermen were designed for a single purpose, to operate the propulsion system on the planet’s surface and steer Mondas away from the dangerous nebula. Only fully-processed Cybermen can survive on the planet’s surface, and even then the survival rate is only 19 per cent; thus, more and more innocent people are being converted into Cybermen and sent to die on the surface. The future seems inevitable after all. The Doctor manages to close most of the generator’s breakers again; they are powered by a geothermal energy source, and if they can get a spark of power flowing to the heat exchangers, the generators will start up again and full power will soon be restored. However, the Doctor refuses to close the final breaker until the Committee agrees to give top priority to restoring power to the people in the city. The Committee agrees, but the final breaker has jammed -- and as the Doctor struggles to close it, Zheng throws the power switch, completing the circuit through the Doctor’s body, restarting the generators, restoring the Committee to full power, and apparently killing the Doctor.

Yvonne’s brother and father are appalled by what’s happened to her; she doesn’t even seem to remember who they are any more. As the power comes back on, the processing of the recruits resumes, and the Cyberman that was Yvonne suffers a seizure, receiving radio signals it isn’t equipped to deal with yet. It finally recognizes Frank and its dad as it dies. Frank, badly shaken by what he’s seen, leaves his father to mourn Yvonne while he goes outside to collect himself. Nyssa follows to find that the city roof has given way to the north; the planet’s frozen atmosphere is pouring down on the hydroponics gardens. Even if the people of the city don’t freeze, they’ll soon starve.

In the palace, the processing of the recruits resumes -- and Sisterman Constant, screaming and protesting all the way, is swept down the conveyer belt to face the same fate as all her other recruits. Meanwhile, the Committee prepares to resume tests of the propulsion system, and, concluding that Allan has proven to be unreliable, plans to place Zheng in charge. Unaware of this, Allen finds to her surprise that the Doctor is still alive and recovering; even Zheng couldn’t survive a blast like that. Allan thus orders Zheng to help her take the Doctor to the wards for a full diagnostic examination. There, she’s horrified to learn that Constant has been sent for processing -- but she soon has something else to think about, for the initial scans of the Doctor’s physiognomy are very interesting indeed.

Allan is convinced that hers is the only way forward. The people of Mondas are dying; they’ve been living underground for so long that just the sight of the open sky drives them mad. The Cybermen are their only hope for survival. But unbeknownst to Allan, the Committee has come to similar but chillingly different conclusions. Although the generators are back online, they don’t yet have enough power to test the propulsion system and repair the city roof at the same time. If they use their resources to prepare the roof, Mondas will travel further into the nebula and the turbulence will grow worse, risking the planet’s destruction; however, if they do not repair the roof, the people of the city will die. There is only one course of action open to them; in order to survive, all citizens of Mondas must undergo full Cyber-processing.

Nyssa is brought in for questioning, and the Doctor tries to convince Allan that Nyssa has the expertise and knowledge to teach Allan how to grow new organs from stem-cell tissues, obviating the need for Cybermen at all. However, Zheng has been given other instructions, and when Allan wavers, he seizes command and sends the Doctor into the scanner before Allan can stop him. The Doctor feels the Committee’s mind pummelling at him, trying to wear down his resistance, but they still argue amongst themselves, unable to decide whether the Doctor is a threat or their saviour. Realizing that dissension will lead to extinction, the Committee casts aside the last remnants of individuality, uniting themselves into a single mind -- a Cyber-Planner, devoted to the continuation of the new cybernetic race.

Frank and his father aren’t doing well; Yvonne is dead, Nyssa has been taken, and Frank has now learned to his horror what would have happened if he’d been accepted onto the work crews as he once wanted. A glacier of frozen atmosphere is pouring through the breach in the roof, and a public service announcement directs all citizens to the Committee palace for “shelter”. Frank and his father know just what that means -- the end of the world. An angry crowd assembles in the square but refuses to enter the palace; thus, Zheng sends his Cybermen into the square to drive the citizens in, using their own emotional weaknesses against them. There is now enough power to operate the propulsion system, but the Cyber-Planner’s priorities have changed; its own survival is now paramount, and in order to ensure that no further dissension from the mob threatens its security, it orders Zheng to begin processing the citizens immediately.

Nyssa waits for the scanner to complete its work, but when it opens up, the Doctor is nowhere to be seen. A fresh new Cyberman then passes through the room, having adjusted fully to its processing already; as Allan had hoped, the new template based on the Doctor will create an invincible race of Cybermen. Nyssa is stunned, believing that the new Cyberman was the Doctor -- until the Doctor himself enters the room. In fact, the new recruit, the first Cyberman to be based on the Doctor’s template, used to be Thomas Dodd. Allan is drinking heavily, having finally foreseen the end result of her work; she’d only ever intended Cyber-processing to be used on selected individuals in an attempt to save her race, but instead she will bear the responsibility for turning her people into an army of animated corpses. The Doctor and Nyssa, determined to stop the Committee, decide to try spiking its nutrient feed with the wine which Allan found in the palace’s old cellars. Allan, drunkenly convinced that the Committee will listen to her, leaves to warn the Committee of the danger they face. On her way, she runs into Zheng, who has just received a report from the surface; the turbulence is increasing, and the propulsion systems must be activated immediately.

The Cybermen herd all civilians towards the processing chambers, but Frank and his father slip away and find the Doctor and Nyssa pouring wine into the Committee’s nutrient feed. The Doctor and Hartley set off to see what further damage they can do, but Frank lags behind to apologize to Nyssa for his childish behaviour earlier -- and they are both captured by Cybermen and taken to the processing lines. Meanwhile, Allan tries to warn the Cyber-Planner that its nutrient line has been contaminated, but the Planner will not listen. The Cybermen drag Allan off to the processing lines as well, and she ceases protesting, giving up all hope. Her creations are pursuing their own vision of survival now, and the people of Mondas are doomed to become an army of the walking dead.

The Cyber-Planner is now diverting power from the propulsion units to the processing chambers to protect itself from the “irrational” citizens of Mondas. The Doctor explains the problem to Hartley, who suggests altering the frequency of the power circuits feeding the Planner; if done properly they could attract a swarm of Cybermats, turning the Cyber-Planner into the biggest “cheeser” ever. The Doctor emerges from hiding and lures away the Cyberman on guard, enabling Hartley to get to work while the Doctor confronts the Cyber-Planner. Its faculties are failing due to the Doctor’s earlier sabotage, and it rejects the Doctor’s assertion that humans and Cybermen alike will be destroyed unless the propulsion system is activated immediately.

Zheng accepts the Doctor’s logic, but the Cyber-Planner guns him down when he attempts to divert the power. Confused and disoriented, the Planner recalls the presence of a twin planet in Mondas’ home star system -- one which expelled a moon from its turbulent surface, throwing off the delicate gravitational balance and causing Mondas to leave orbit. As a result, the twin thrives while Mondas faces extinction. As the Planner ponders what to do about this, a swarm of Cybermats, attracted by the energy pulse Hartley has generated, attack the Planner, disorienting it just long enough for the critically damaged Zheng to divert power away from the Planner to the propulsion systems. The Planner shuts down, presumably forever, and Mondas’ course changes, taking it out of the nebula to safety.