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In the conclusion of the six-part story "The Mind of Evil," the Master (Roger Delgado) has launched a nerve-gas missile, armed with the combined negative emotions of a group of imprisoned criminal and aimed at a world peace conference. The Doctor (Jon Pertwee) hurriedly attempts to deactivate the hate-inducing weapon -- and also endeavors, yet again, to put the Master out of business once and for all. The huge sets and props utilized in this story arc pushed the otherwise parsimonious Doctor Who budget far beyond its usual boundaries. Written by Don Houghton, "The Mind of Evil, Episode 6" first aired on March 6, 1971. Originally filmed in color, this episode currently exists only in a black-and-white version.