The TARDIS is lured to Earth in 1985 by a distress call sent by Lytton, who has made contact with a group of Cybermen based in London's sewers. The Doctor and Peri are then captured and forced to take Lytton and the Cybermen in the TARDIS to the Cybermen's home planet Telos. The Cybermen have stolen a time vessel from another race and plan to change history by crashing Halley's Comet into Earth and obliterating it before it can bring about the demise of their original home world, Mondas, in 1986.

Lytton is, however, a double agent employed by the Cryons - a species native to Telos. His mission is to capture the stolen time vessel, but he fails and is partially converted into a Cyberman. The Doctor is unable to save him, but manages to kill the Cyber Controller. The Cryon leader Flast sacrifices her own life and a huge explosion completely destroys the tombs.