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The Doctor locates the sixth and last missing segment of the Key to Time, which is necessary to restore the balance between Good and Evil in the universe. Unfortunately, the Doctor (Tom Baker) and Romana (Mary Tamm) may not be able to retrieve the precious segment -- not if the much-dreaded (and still unseen) Black Guardian has anything to say about it. This final adventure of Doctor Who's 16th season began its run on January 20, 1979, and lasted six episodes, bringing the series' entire episode count to a daunting 504. "The Armageddon Factor" was written by Bob Baker and David Martin.