In the conclusion of the four-part story "Terror of the Zygons," the Doctor (Tom Baker) is captured by the alien Zygons, who intend to sabotage the World Peace Council as their first step in conquering the Earth. Despite his comparative helplessness, the Doctor tries to save London from the Zygon assault, while the Brigadier (Nicholas Courtney) offers "outside" assistance. This episode represented the final regular Doctor Who appearance of Ian Marter as Harry Sullivan (though Marter would make a return guest appearance in "The Android Invasion"). First shown on September 20, 1975, "Terror of the Zygons, Episode 4" was written by Robert Banks Stewart. ~ Hal Erickson, Rovi