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In the conclusion of the four-part story "Day of the Daleks," the 22nd century Ogrons prepare to journey back in time to assassinate Sir Reginald Styles (Wilfred Carter), the diplomat whom they hold responsible for plunging the world into nuclear war, thereby paving the way for a Dalek takeover. But the Doctor (Jon Pertwee) discovers that Styles was not the culprit; instead, it was a renegade Ogron who caused all the trouble. Now the Doctor must not only save Sir Reginald, but also alter the course of the future. First telecast January 22, 1972, "Day of the Daleks, Episode 4" was written by Louis Marks.