Returning to N-Space, The Doctor and Adric are summoned to Traken, a planet of peace and harmony. The Keeper of Traken is nearing the end of his 1000-year-old reign and one of the Traken consuls is to become the next Keeper. The Doctor, Adric, Tremas (the Keeper Nominate) and Nyssa (his daughter) set about to stop the Melkur, a calcified figure believed to be a "Source of Evil" trying to destroy the Traken Union, only to later discover it is the guise of a old enemy of the Doctor - The Master - who plans to take control of The Source (the keystone of Union) so he can regenerate into a new body.