The Doctor is invited to visit the planet Tigella by its leader, Zastor, who has become concerned about disputes between his people's two opposing factions, the religious Deons and the scientist Savants. The TARDIS is intercepted by the megalomaniacal xerophyte Meglos, last survivor of the planet Zolfa-Thura, and trapped in a chronic hysteresis - a time loop - but the Doctor and Romana manage to free it.

The Eternals are having a space race, but one works for the Black Guardian.

The final segment of the Key is traced to the planet Atrios, engaged in a long war with the neighbouring Zeos. The Marshal of Atrios intends a final strike to destroy the Zeons, but the Doctor and Romana discover that Zeos is deserted and the war is being co-ordinated by a computer called Mentalis, built by one of the Doctor's old Time Lord Academy friends, Drax.

The final segment of the Key is traced to the planet Atrios, engaged in a long war with the neighbouring Zeos. The Marshal of Atrios intends a final strike to destroy the Zeons, but the Doctor and Romana discover that Zeos is deserted and the war is being co-ordinated by a computer called Mentalis, built by one of the Doctor's old Time Lord Academy friends, Drax.

The Tardis materializes on what appears to be an Edwardian racing yacht.

Turlough tries to commit suicide to escape the influence of the Black Guardian.

The doctor's (Peter Cushing) TARDIS time machine takes him, his granddaughters and a friend (Roy Castle) to war-ravaged planet Skaro.

Dr. Who (Peter Cushing), his granddaughter, his niece and a London bobby (Bernard Cribbins) find that Daleks have invaded 22nd-century Earth and are turning humans into slaves.