In the conclusion of the four-part story "The Android Invasion," the Doctor (Tom Baker) and Sarah (Elisabeth Sladen) have arrived back on Earth, to warn the populace that an army of androids, controlled by the Kraals, are about to infect the planet with a deadly virus. Alas, the Doctor's UNIT contact, the Brigadier (Nicholas Courtney), is out of town, so the warnings go unheeded. Only the Doctor's expertise in the field of electronics stands between Mankind and the Kraals. Written by Terry Nation, "The Android Invasion, Episode 4" originally aired on December 13, 1975. ~ Hal Erickson, Rovi