30 January 1971
Roger Delgado (The Master), Nicholas Courtney (Brigadier Lethbridge-Stewart), Richard Franklin (Captain Mike Yates), John Levene (Sergeant Benton) [2-6], Pik-Sen Lim (Captain Chin Lee) [1-3], Raymond Westwell (Prison Governor) [1-3], Michael Sheard (Dr. Summers) [1-4, 6], Simon Lack (Professor Kettering) , Neil McCarthy (Barnham) [1-3, 5-6], Fernanda Marlowe (Corporal Bell) [1-4], Clive Scott (Linwood) , Roy Purcell (Chief Prison officer Powers) [1-3], Eric Mason (Senior Prison officer Green) [1-3], Bill Matthews, Barry Wade, Dave Carter, Martin Gordon (Prison officers) [1-4, 6], William Marlowe (Mailer) [2-5]*, Haydn Jones (Vosper) [2-4], Kristopher Kum (Fu Peng) [2-3], Tommy Duggan (Senator Alcott) &dagger, David Calderisi (Charlie) , Patrick Godfrey (Major Cosworth) [5-6], Johnny Barrs (Fuller) , Matthew Walters (Main Gates Prisoner)
|Written by||Don Houghton|
|Directed by||Timothy Combe|
|Produced by||Barry Letts|
UNIT is handling the security for the World Peace Conference in London, but the Master is plotting to plunge the world into war by wrecking it.
Posing as Emile Keller, a Professor of Criminology, he goes to Stangmoor Prison in England with a machine that apparently removes the negative (or evil) impulses from the minds of hardened criminals. Actually, the Keller machine is a weapon: it contains an alien mind parasite that stores, and feeds on, these evil impulses.
Unsuspected because of his forged credentials, he secretly supplies guns to the prisoners, and uses the mind parasite to start a riot by stirring up hostility among them, in which they take control of the prison. He plans to use them to seize a nerve gas missile being transported by UNIT, with which he hopes to destroy the Peace Conference.
In case his plan to hijack the missile is thwarted, he intends to use the mind parasite (once it has fed sufficiently) to wreck the Peace Conference by murdering the American and Chinese delegates, who are to be killed by the mind parasite’s telepathic powers.