In the conclusion of the four-part story "The Visitation," the Doctor (Peter Davison) squares off against the alien Tereleptils, who hope to capitalize upon the Black Plague sweeping through 17th century London by spreading a deadly viral epidemic throughout the rest of the world. This episode proposes to answer the age-old question: Who really ignited the Great Fire of London? Written by Eric Saward, "The Visitation, Episode 4" first aired on February 23, 1982. ~ Hal Erickson, Rovi