 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.