Serial: FFF  Episode Nos. 283, 284, 285, 286, 287, and 288.  Title: The Mind of Evil

Capital offenders are being treated experimentally by the Keller Machine, a device that extracts the evil from men's minds.  The Doctor has grave misgivings about the machine's true nature, especially when people who go too near it begin to die.  Meanwhile, UNIT is handling the security for a World Peace Conference and also transporting a nerve gas missile for disposal.  These three apparantly unrelated events are in fact all being linked by the machinations of the Master...

starring Jon Pertwee as Doctor Who, Roger Delgado as the Master, Nicholas Courtney as Brigadier Lethbridge-Stewart, Katy Manning as Jo Grant, Richard Franklin as Captain Yates, and John Levene as Sergeant Benton.. Written by Don Houghton, Produced by Barry Letts, and Directed by Timothy Combe.

Originally transmitted from 30 January 1971 to 6 March 1971 on BBC1.