While Biroc leads the Doctor to a view of the Tharil's lordly past, Romana learns more about the damaged freighter and the coldheartedness of its crew. Meanwhile Commander Rorvik, confounded by the time mirror inside the universal center gateway, ignores evidence that it's no simple mirror and decides to blast it, imperiling the lives of everyone.

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.