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In the third episode of the six-part story "The Mind of Evil," the Doctor (Jon Pertwee) once more crosses the path of his arch-rival, the Master, who has assumed the guise of mild-mannered Professor Emil Keller, and infiltrated a world peace conference. It doesn't take long for the Master to wreak his usual havoc, this time by utilizing a mind-altering nerve gas. The Doctor knows what is going on, but he still doesn't know why. Written by Don Houghton, "The Mind of Evil, Episode 3" first aired on February 13, 1971. Originally filmed in color, this episode currently exists only in a black-and-white version.