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In the fourth episode of the six-part story "The Armageddon Factor," the Doctor (Tom Baker) is still on the planet Atrios, still determined to locate the sixth and final segment of the Key to Time. In this pursuit, he has offered his aid to Princess Astra (Lalla Ward) in Atrios' war against its twin planet Zeos. Gumming up the works is a secret henchman of the Black Guardian, who has vowed that the Key to Time will never be put together again. The henchman is hiding on a mysterious, hidden Third Planet between Atrios and Zeos -- and it is up to the Doctor to locate that planet. Written by Bob Baker and David Martin, "The Armageddon Factor, Episode 4" originally aired on February 10, 1979.