Doctor Who: The Mind of Evil

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.