The Armageddon Factor is a serial in the British science fiction television series Doctor Who, which was first broadcast in six weekly parts from January 20 to February 24, 1979. It is the last story in The Key to Time arc, and the last to feature Mary Tamm as Romana. The quest for the final segment to the Key to Time brings the Doctor and Romana to Atrios, a world caught in a perpetual, stalemated war with its planetary neighbour Zeos.

But the Black Guardian is closing in...In their search for the final segment of the Key to Time, the Doctor and Romana arrive on the planet Atrios, which has endured a recent bombing by their neighbouring planet Zeos, with whom they are at war. The Marshal of Atrios is about to launch a counterstrike on Zeos, but the Doctor finds that Zeos is deserted save for the giant computer Mentalis, which is controlling the war.

He also discovers that the true opponent is a third planet called the Planet of Evil, ruled by "the Shadow". The Shadow, an agent of the Black Guardian, has Princess Astra of Atrios captive, threatening to torture her if she doesn't give him the location of the final segment of the Key to Time (which she doesn't know). After disabling Mentalis, with the assistance of Romana and K-9, the Doctor creates a substitute sixth segment out of chronodyne, which gives the Key enough power to create a time loop in which to trap the Marshal's ship (armed with missiles for the final strike) as well as the Mentalis control room (engaged in an automatic self-destruct sequence).

On the Planet of Evil, the Doctor encounters an old Academy mate, a fellow Time Lord named Drax, who has been employed under duress by the Shadow but agrees to help the Doctor.

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