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In the fifth episode of the six-part story "The Invasion of Time," the Doctor appears to be cooperating with the telepathic Vardans in their efforts to conquer his home planet Gallifrey. But just when victory seems within the Vardans' grasp, the Doctor springs the trap that he has planned all along. Alas, Gallifrey now faces a threat from an even more formidable enemy -- the Sontarans. First telecast March 4, 1978, "The Invasion of Time, Episode 5" was written by Graham Williams and Anthony Read, under the joint pen name of David Agnew.