The Fourth Doctor answers a distress signal from Professor Chronotis, a Time Lord posing as a professor at St. Cedd's College, Cambridge who loaned a Gallifreyan tome to his student Chris Parsons. The Doctor retrieves the book while Chronotis dies after his mind was extracted by the sphere of a mad scientist named Skagra, living long enough to warn Romana, K9 and Parsons of them and Shada. The Doctor locates Skagra's cloaked spacecraft, only for his companions to be captured while Skagra has his sphere extract the Doctor's mind to decode the book before taking Romana in the TARDIS to his carrier ship and Krarg creations. But the Doctor survived his ordeal with his mind intact and has the ship's computer release Chris and K9 and take them to a space station Skagra previously occupied. The group find Skagra's discarded colleagues and learn he is after a Time Lord named Salyavin.