In the final episode of the four-episode story arc "The Aztecs," Barbara's misguided efforts to alter the course of history have discredited her in the eyes of the Aztec worshippers, who no longer believe that she is the reincarnation of the high priestess Yetaxa. Consequently, Barbara, Susan, and Ian are slated to be "guests of honor" at a human sacrifice. The Doctor (William Hartnell) endeavors to rescue his companions -- with the unexpected assistance of a total eclipse. Written by John Lucarotti, "The Day of Darkness" first aired on June 6, 1964. ~ Hal Erickson, Rovi