The Armageddon Factor is the sixth and final serial of the 16th season of the British science fiction television series Doctor Who, which was first broadcast in six weekly parts on BBC1 from 20 January to 24 February 1979. It was the last to feature Mary Tamm as Romana. The serial is set on the planets Atrios and Zeos, and another planet in between them. In the serial, the war between Atrios and Zeos is nearing its end. Mentalis, a commandant computer on Zeos, and the Marshal of Atrios (John Woodvine), plot to wipe each other out. At the same time, the Shadow (William Squire), a henchman of the Black Guardian (Valentine Dyall), steals the time and space vessel the TARDIS, which contains the first five segments of the powerful Key to Time, and kidnaps Princess Astra of Atrios (Lalla Ward), who is the sixth and final segment. Plot Searching for the final segment of the Key to Time, the Fourth 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 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. On the Planet of Evil, the Doctor encounters another Time Lord, Drax, who he last met at the Academy. Drax has been employed under duress by the Shadow but agrees to help the Doctor. The Doctor leads a servant of the Shadow, known as a Mute, to his TARDIS and opens the door, but is suddenly shrunk to tiny size by Drax, who then shrinks himself using the dimensional stabiliser from his own TARDIS. Drax had misinterpreted the Doctor's plan and shrank the Doctor instead of the Mute. The Mute returns to the Shadow with the Key, and the Doctor realises why the Shadow has requested it: Astra is the final segment, and is transformed in front of everyone. Using their diminished size, the Doctor and Drax smuggle themselves into the Shadow's lair inside of K9, who pretends to still be under