Parts: 4
Writer: Philip Martin
Airdates: 4 October 1986 - 25 October 1986
Production Code: 7A
Alternate Title: The Trial of a Time Lord parts Five through Eight
Megalomaniacs of the Week: the Valeyard, Sil
Special Guests: the Time Lords
Story: The Valeyard continues his prosecution with the adventure in which the Doctor was involved immediately before he was snatched to the trial. Investigating the words of a dying warlord, the Doctor brings Peri to Thoros-Beta, a planet of green sky, blue rocks and sludgy pink ocean. Attacked by a hideous guard-creature, then menaced by humanoid guards and nearly savaged by a lycanthrope, the Doctor and Peri are quite bewildered as to the true nature of the planet. And then Sil turns up.
Thoros-Beta is Sil's homeworld, as Peri soon learns, and she wants away from the dreadful planet. But the Doctor is keen on investigating, and he finds a laboratory being used for brain experiments. Sil's master, the Lord Kiv, is suffering from brain swelling, and for him to survive, his mind must be transplanted into a new body. And Sil thinks he has the perfect solution...
Peri's trust in the Doctor plummets when, following a brain scan, he appears to join forces with Sil, even going so far as to betray her. Thanks to the memory-wiping effects of the brain scan, the Doctor watching in the courtroom has forgotten what happens next...and he has a very nasty surprise waiting for him.