Returning to N-Space, The Doctor and Adric are summoned to Traken, a planet of peace and harmony, where The Keeper of Traken is nearing the end of his 1000 year old keeper-ship and one of the Counsels of Traken is to become the next Keeper. Where The Doctor, Adric, Tremas, one of the Counsels elected to become Keeper and Tremas's daughter Nyssa set out to stop the evil Melkur, believed to be a "Source of Evil" that is trying to destroy the Union. Only to discover Melkur is being used by a old enemy of the Doctor, The Master, who plans to take control of The Source, The Union's keystone, so he can regenerate into a new body.