Setting the controls for Earth, the Fourth Doctor is surprised when the Tardis lands in a primeval forest. Has the Tracer gone wrong or has some impulse deep in the unconscious mind directed him to this alien planet? In investigating the forest, the Doctor meets and assists Leela, a warrior banished from her tribe, the Sevateem. Through Leela, it gradually becomes apparent that the constant war between the Sevateem and the Tesh has been instigated by the god they both worship, Xoanon. Xoanon, an all-powerful computer, is possessed by a desperate madness – a madness that is directly related to Doctor Who, that causes Xoanon to assume the voice and form of the Doctor, a madness that is partly caused by the Doctor and that only the Doctor himself can rectify! The Doctor must not only do battle with Xoanon, but also must escape from the savage practices of the Sevateem, and the technically mind-controlling destructive impulses of the Tesh.