The Doctor and Romana arrive in present-day Cambridge to visit an old friend of the Doctor's, Professor Chronotis, who has rooms at St Cedd's College. However, the Doctor is not the only one with an interest in the Professor, as a criminal called Skagra is also after him.

Skagra has a mind-draining sphere in which he hopes to trap the Professor's mind. He is also after a book which the Professor has, The Ancient and Worshipful Law of Gallifrey. The book, however, has been borrowed by mistake by student Chris Parsons and Skagra leaves empty-handed.

Skagra eventually manages to get the book, after his sphere chases the Doctor through the streets of Cambridge. He also kidnaps Romana. The book is the key to how to get to Shada, the ancient prison planet of the Time Lords. Skagra wants to obtain the mind of a criminal called Salyavin who was incarcerated there.

It transpires that Chronotis is Salyavin and that he escaped from Shada years ago and has been living peacefully ever since. The Doctor prevents Skagra's plans for domination by winning a mind battle against him and his alien allies the Krargs.

_{Synopsis from Doctor Who: The Fourth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}