The TARDIS is lured to Earth, 1985, by an alien distress signal. Investigating, the Doctor and Peri stumble upon an attempted bank robbery staged through the London sewers by a familiar foe -- ex-Dalek agent, Lytton. But who is he working for this time? For they soon discover there is something far nastier lurking in the sewers -- Cybermen. Why do the silver giants need a time machine? What is their devastating plan of attack? And what does it have to do with a catastrophic event that happened long ago in the Doctor's past?