The Sevateem are preparing an assault against their enemies, the Tesh, to free their god Xoanon. One of the warriors, Leela, argues against the attack and is exiled. Leela meets the Doctor, who looks like the Evil One of the Sevateem religion. The Doctor works out that the relics and riterals of the Sevateem that they are the descendants of the Earth Survey Team, while the Tesh are derived from the technicians who remained with he ship. Xoanon turns out to be a computer the Doctor once tried to repair. The Doctor's memory trace drove the computer mad, causing it to manipulate the Sevateem and the Tesh into a war. The Doctor and Leela try to reach Xoanon and remove the Doctor's personality from it. Xoanon, whose view of reality is threatened by the Doctor's existence, uses mind control to send Sevateem ad Tesh alike on a single mission: kill the Doctor.