1972-01-22

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.

BBC