In the fifth episode of the six-part story "Colony in Space," the Doctor (Jon Pertwee) tries to persuade the Guardian (Norman Atkyns) of the planet Uxarius to destroy a doomsday machine that is worshipped as a god by the local natives. But he is thwarted in this pursuit by the Master (Roger Delgado), who intends to use the machine for his own nefarious purposes. Written by Malcolm Hulke, "Colony in Space, Episode 5" first aired on May 8, 1971. ~ Hal Erickson, Rovi