Traken is a planet ruled by the all-powerful Keeper, who reigns for a thousand years and ensures that his subjects live a life of tranquillity and harmony, free from the forces of evil and corruption. Now the Keeper is dying. Summoned by the ailing Keeper, the Doctor and Adric discover that all is not well on Traken. An evil presence threatens the future of the planet - an evil that knows all about the Doctor.