In the sewers of London, workers are attacked by an invisible force. The Doctor (Colin Baker) is trying to repair the Chameleon Circuit of the TARDIS, but this causes it to behave strangely. It lands on Earth in 1985, where he shows Peri (Nicola Bryant) Halley’s Comet.
Meanwhile, the former Dalek mercenary Lytton (Maurice Colbourne) is now a London gangster and is planning a ten-million-pound diamond heist with his mob Griffiths (Brian Glover), Payne (James Beckett), and Russell (Terry Molloy). They intend to enter the bank through the sewers, but Lytton activates a transmitter first which emits a distress signal the Doctor picks up, and the TARDIS lands in a scrapyard in Totter’s Lane. It changes to an ornamental dresser. Trying again, the Doctor lands the TARDIS in the sewers as an organ. After pausing to play a little Bach, he finds the transmitter but is held up by two policemen under Lytton’s control.
In the sewers, Payne falls behind the others and is beaten to death by the force that had attacked the workers. Lytton and the others come to a dead end and are approached by a Cyberman. Griffiths shoots it, but Lytton disarms him and surrenders. The Cybermen have established a base in the sewers. Russell flees, finds the Doctor and Peri and tells them he is an undercover officer.
Meanwhile, on the Cybermen’s planet Telos, two slaves, named Bates (Michael Attwell) and Stratton (Jonathan David), escape from a work-gang and decapitate a Cyberman. They use its helmet to disguise Stratton and enter Cyber Control, looking for their time-machine. The Doctor, Peri, and Russell return to the TARDIS, where they are ambushed by the Cybermen. The Cyberleader threatens Peri.
The Doctor threatens to self-destruct the TARDIS if they do not release her. The Cyberleader agrees and tells the Doctor that the Cyber Controller, whom the Doctor thought he killed a long time ago, is still alive on Telos. He is forced to take the TARDIS to Telos, and all are imprisoned. The Doctor tells the others about the Cryons, wiped out so the Cybermen could use their Cryogenic technology. He wonders why Lytton is so familiar with Cyberman history and becomes suspicious.
On Telos, most of the Cybermen are brain-damaged and go on destructive rampages. The TARDIS arrives in the depths of the cryogenic tombs. One of the Cybermen goes on a rampage and everyone but the Doctor manages to escape. Peri, Lytton, and Griffiths meet Cryons who are not extinct after all and have been giving the Cybermen troubles. Lytton is working for them. He offers Griffiths two million pounds in diamonds (common on Telos) to help capture the time-machine. They track down Bates and Stratton, whose arms and legs have been replaced. They agree to work together.
The Doctor is imprisoned in a cold storage room with Flast (Faith Brown), former leader of the Cryons. She tells the Doctor that the Cybermen plan to go back in time and prevent the destruction of their original homeland Mondas (which the Doctor had a hand in long ago) by directing Halley’s Comet into the Earth before Earth can attack them. The Doctor realizes he has been sent by the Time Lords to prevent this. The storage room contains vastial, which can be explosive. The Doctor uses some to dispose of a guard, then gives Flast, who is unable to leave the cold room, a sonic lance before he escapes. The Cybermen arrive and kill her, but the lance is already in the vastial.
Lytton, Griffiths, Bates, and Stratton get to Cyber Control, but Lytton is captured. He is tortured to reveal the plans, but to no avail, so they decide to make him a Cyberman. The other three get as far as the landing pad but are killed. The Doctor gets to the TARDIS and the Cryons give him Peri. They reveal that Lytton was working for them and the Doctor agrees to try to save him. The TARDIS arrives in Cyber control, where they find a partially converted Lytton. The Doctor tries to free him, but the Controller arrives with a gun. Lytton attacks the Controller, who kills Lytton. The Cyberleader and his lieutenant arrive, but kill each other in a crossfire, and the Doctor grabs a gun and kills the Controller. The Doctor escapes with Peri. The sonic lance in the vastial material ignites, setting off a series of explosions that destroy Cyber Control and the tombs. The Doctor feels bad about misjudging Lytton so badly.
The repair of the Chameleon Circuitry was supposed to be permanent, but that didn’t happen for the same reason it stopped working in 1963: it would have been expensive. The production did succeed in forcing the lighting crew to keep the sewers dark and creepy, however. The director was praised, but the story was considered complex and derivative. It was thought to be a rehash of previous stories, including Tomb of the Cybermen, with the Second Doctor. But Colin Baker was praised for his acting. Instead of being broadcast in four 25-minute episodes, it was broadcast in two 45-minute episodes, and the show remained that way for the rest of the season. When the TARDIS appeared in the junkyard, the music was that of Steptoe and Son, also written by Ron Grainer. The story was criticized for its violence, especially the torture scene. Maurice Colbourne as Lytton is quite effective—his cold, steely-eyed gaze is frightening--and his transformation into a Cyberman is horrific. Brian Glover and James Beckett as his hapless henchmen acquit themselves well, as does Terry Molloy, who has usually been unrecognizable in the Davros costume. As usual, Doctor Who is blessed with a fine supporting cast.