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In the third episode of the four-part story "The Visitation," the Doctor (Peter Davison) and his companions find themselves in the London of 1666 at the time of the Great Plague. At the same time, a group of alien Tereleptils intend to take advantage of the chaos and panic by unleashing an even deadlier viral epidemic throughout the earth in hopes of conquering the planet themselves. Written by Eric Saward, "The Visitation, Episode 3" first aired on February 22, 1982.