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.