When the Doctor lands on a space station in the Third Zone, he suspects he's been there before and when he discovers his old assistant, Jamie, he's certain. Accompanied by Jamie and his present assistant Peri, the Doctor follows himself to 20th century Seville. There he discovers that his old enemies, the Sontarans, are about to dissect him in a genetic operation.

The TARDIS is lured to Earth, London, 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...