On a nameless planet in the far future where invisible monsters prowl, the Doctor befriends a warrior woman named Leela, recently banished from her tribe, the Sevateem. She and everyone recognize the Doctor immediately. He is the Evil One who, in their mythology, holds their god Xoanon prisoner and must be destroyed.

The Doctor and Leela encounter the placid and mind-powerful Tesh, the complete opposite of the savage Saveteem. They recognize the Doctor, too, but to them he is Lord; however, when it becomes evident the Doctor threatens their living god, Xoanon, the Doctor becomes a marked man.