The Penultimate episode of Tom Bakers final season as the Doctor and the return of an old foe. Introducing Sarah Sutton as Nyssa. The Doctor and Adric learn from the wizened Keeper of Traken that a great evil has come to his planet in the form of a Melkur - a calcified statue.

The Keeper is nearing the end of his reign and seeks the Doctor's help in preventing the evil from taking control of the bioelectronic Source that is the keystone of the Traken Union's civilisation. The Melkur, via various deceptions, becomes the next Keeper, hopes to use the Source's power to regenerate himself.