On the edge of the universe, the Doctor, Leela and K9 encounter a Minyan ship on an epic quest to find their race banks... but their people have encountered the Time Lords before...

The Doctor and Romana arrive on the marsh moon of Delta Magna in search of the next segment of the Key to Time. They are caught in the conflict between the native Swampies and the crew of a chemical refinery. The presence of a gun runner complicates matters; to make things worse, the Swampies intend to awaken Kroll, the giant god that lives beneath the swamps.

The TARDIS brings the Doctor, Susan, Barbara, and Ian to the planet Skaro where they meet two indigenous races — the Daleks, malicious mutant creatures encased in armoured travel machines, and the Thals, beautiful humanoids with pacifist principles. They convince the Thals of the need to fight for their own survival.

Guerillas time-jump back from the 22nd century to assassinate Sir Reginald Styles - a diplomat working for World Peace, The Doctor discovers they are from a future where the Daleks have invaded the Earth (Following wars precipitated by Styles's destruction at the peace conference). With the Daleks desperate to maintain their version of history, The Doctor and Jo find themselves in a totalitarian future, where they must discover who really happened and prevent it.