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.