The TARDIS arrives on an unnamed planet where a tribe of savages, the Sevateem, worship a god called Xoanon. Xoanon speaks to them through their holy man, or shaman, Neeva, and he exhorts them to find a way through a deadly energy barrier which separates them from the forbidden land beyond, which is controlled and enjoyed by the Tesh.

The Doctor finds that his face is also the face of the Evil One - indeed there is a massive carving of it on a cliff near the Sevateem's village. Only one tribe member feels that the Doctor is good. This is Leela, cast out from the tribe for daring the question the wisdom of Xoanon.

Leela helps the Doctor to investigate further, and he discovers that he can get through to the other side of the time barrier by way of passages carved out of the cliff, behind the stone image of his own face.

Xoanon is a super-computer, which arrived on the planet when a space craft from the Mordee Expedition crashed there many years ago. Some of the ship's crew were originally to have surveyed the world for possible colonisation, but then the Doctor arrived on an earlier visit and helped the technicians to repair the computer. Unfortunately he neglected to erase his own personality print which subsequently made it schizophrenic.

The computer then arranged for the crew to be split up: the technicians (Tesh) on the one side and Survey Team 6 (Sevateem) on the other. It began an experiment, raising the Tesh as ascetic telepaths who would tend with religious fervour to its every need, while allowing the Sevateem to descend into savagery.

The Doctor realises his original mistake, and manages to use the reverse memory transfer system to wipe the additional personalities from the computer, leaving it sane and in proper control once more.

Leela, deciding that she no longer wants to stay on her own planet, pushes her way into the TARDIS much to the Doctor's chagrin. It seems he has gained another companion.

_{Synopsis from Doctor Who: The Fourth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}