The Doctor and Romana visit Chronotis, a retired Time Lord living in Cambridge, who admits that he's lost a book dating back to the time of Rassilon and needs their help to find it. In fact he's accidentally lent it out to graduate student Chris Parsons, who soon realises that this is an extraordinary text indeed. The book is also sought by the evil scientist Skagra, who has developed a sphere capable of drawing its victims' minds out of their bodies. Skagra needs the book to reach the Time Lord prison Shada, which even the Time Lords have forgotten. Long ago they imprisoned the criminal Salyavin there, as punishment for using his powers to transfer his mind into other people's bodies. Chronotis is eventually forced to admit that he himself is Salyavin; he escaped long ago and used his powers to cause the Time Lords to forget about his prison. However, he's now repented of his crimes and wishes to live a quiet, peaceful life. Skagra drains Chronotis's mind and thus acquires the ability to transfer his own mind into other bodies; with the aid of the sphere and his crystalline servants, the Krargs, he will spread his own mind throughout the Universe and become immortal. However, when the Doctor pretends to be stupid, the sphere fails to drain his mind completely. He thus survives, and since there's a copy of his mind inside the sphere, he is able to use the resonance to turn Skagra's possessed slaves against him. Skagra is imprisoned by his own Ship, and Chronotis returns to Earth to resume his peaceful retirement.