In the fifth episode of the six-part story "Dalek Invasion of Earth," the Doctor (William Hartnell) has finally tumbled onto the Daleks' scheme to convert the earth into a huge space vessel. Racing against time, the Doctor must find a way to foil the mutants' plans. He receives help in this respect from a most unexpected source -- but is it already too late to save the world? Written by Terry Nation, "The Walking Ally" first aired on December 18, 1964. ~ Hal Erickson, Rovi