Mindwarp

From: http://www.bbc.co.uk/doctorwho/classic/episodeguide

The Valeyard's second segment of evidence relates to the planet Thoros-Beta. Here the Doctor and Peri meet their old adversary Sil and others of his Mentor race, whose leader Kiv is awaiting an operation from a scientist named Crozier to transplant his brain into another body. They also form an uneasy alliance with a kidnapped Krontep warrior, King Yrcanos, and encounter a group of resistance fighters. Peri is eventually chosen as the recipient of Kiv's consciousness and is apparently killed in an ensuing mêlée sanctioned by the Time Lords to prevent Crozier's work from disturbing the balance of nature.

24 out of 26 (92.3%) raters say this story requires a previous story.