Doctor Who: The Armageddon Factor, Episode 6 on AllMovie

In the conclusion of the six-part story "The Armageddon Factor," the Doctor (Tom Baker) has discovered that Princess Astra (Lalla Ward) of the planet Atrios is, in fact, the much sought-after sixth element of the Key to Time. Armed with this knowledge, the Doctor and Romana (Mary Tamm) are at last able to reassemble the Key, thereby restoring the balance between universal Good and Evil. The one remaining fly in the ointment is the dreaded Black Guardian, who has a vested interest in upsetting that aforementioned balance. Capped by a surprising but logical ending (logical by the Doctor's standards, at any rate), "The Armageddon Factor" was written by Bob Baker and David Martin, and originally aired on February 24, 1979, as the final episode of Doctor Who's 16th season.