The TARDIS lands on a planet the Doctor doesn't recognise - but its inhabitants seem to know him. They think he's the Evil One and aren't pleased to see him.

Two warring tribes occupy the planet: the savage Sevateem and the reclusive Tesh, separated from each other by a deadly energy field. When the Sevateem believe their God, Xoanon, to be a captive of the Tesh, they decide to break thriough the barrier and rescue him. The Tesh, however, have other ideas.

Befriended by Leela, a Sevateem girl who dares to question the mighty Xoanon, the Doctor sets out to discover exactly what's happening on this planet. Has he been here before? Is he responsible for the tribal wars? And can it possibly be that Xoanon, the mad God, is really the Doctor?

Originally transmitted 01/01/77 - 22/01/77