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In the fourth episode of the six-part story "The Mind of Evil," the Master (Roger Delgado), renegade Time Lord and perennial nemesis of the Doctor (Jon Pertwee), has revealed his presence at a World Peace Council. Going one better, the Master also reveals his current scheme: By sapping the evil thoughts of criminals, he intends to accumulate enough negative energy to successfully steal -- and activate -- a deadly nerve-gas missile. Written by Don Houghton, "The Mind of Evil, Episode 4" first aired on February 20, 1971. Originally filmed in color, this episode currently exists only in a black-and-white version.