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In the second episode of the four-part story "The Power of Kroll," the Doctor (Tom Baker) and Romana (Mary Tamm) trace the fifth segment of the Key to Time on the swamp moon of Delta Magna. Almost immediately, the two time travellers incur the wrath of the local Swampies, who worship a God called Kroll. In time-honored Doctor Who tradition, Kroll requires a human sacrifice -- and Romana is chosen for that honor. Written by Robert Holmes, "The Power of Kroll, Episode 2" originally aired on December 30, 1978.