'The Armageddon Factor' (6 Parts) Story Code '5F' by Bob Baker and Dave Martin

The search for the final segment of the Key to Time takes the Doctor, Romana and K-9 to the planet Atrios, which is engaged in an ongoing nuclear war with the neighbouring planet of Zeos. The controller of the Atrios defence, the Marshal, intends to launch a final strike against the Zeons, but the Doctor suspects that an outside force is controlling him. The final segment appears to be connected in some way to the missing Princess Astra of Atrios, and the two Time Lords travel to Zeos in search of her. Here they discover that Zeos is instead deserted, and that the war is being co-ordinated by a computer named Mentalis, which is under the control of the Shadow, an agent of the evil Black Guardian. The Shadow and his Mute servants inhabit an unseen third planet, which lies between Atrios and Zeos; the Shadow has kidnapped the Princess, and is using the Marshal to launch a final assault on Zeos that will trigger Mentalis into destroying itself and both planets. The Doctor creates a temporary version of the final segment using chronodyne, and uses the completed Key to place a time loop around the Marshal's ship, before he launches his final attack on Zeos. The Doctor and Romana learn that Drax, one of the Doctor's old Time Lord Academy friends, built Mentalis. The three Time Lords work together, and discover that the real sixth segment is Princess Astra of Atrios, but the Shadow transforms her into the segment itself, and she ceases to exist. The Doctor manages to snatch the segment, and escapes back to the TARDIS, where he finally completes the Key. Drax deactivates Mentalis, and the Doctor redirects the Marshal's attack, which destroys the Shadow and his planet. The White Guardian appears on the ship's scanner screen, and congratulates the Doctor on his success, and asks to take charge of the Key. But the Doctor decides that it is too powerful for any one being to control, and, believing that it has now had time to restore some balance to the universe, he orders it to disperse throughout the cosmos. Enraged, the Guardian reveals himself to be the Black Guardian, and vows to take his revenge on the Doctor. In order to escape his threat, the Doctor fits a Ramdomiser to the TARDIS controls, which will prevent his travels from being forecast.

Tom Baker (Doctor Who), Mary Tamm (Romana), John Leeson (Voice of K9), John Woodvine (Marshal), Lalla Ward (Princess Astra), Davyd Harries (Shapp), Ian Saynor (Merak), Ian Liston ('Hero'), Susan Skipper ('Heroine'); John Cannon, Harry Fielder (Guards); William Squire (The Shadow), Iain Armstrong (Technician), Pat Gorman (Pilot), Barry Jackson (Drax), Valentine Dyall (The Guardian)

Produced by Graham Williams
Directed by Michael Hayes

TX:
Part 1 - 20th January, 1979 @ 6.25pm - 6.50pm
Part 2 - 27th January, 1979 @ 6.25pm - 6.50pm
Part 3 - 3rd February, 1979 @ 6.25pm - 6.50pm
Part 4 - 10th February, 1979 @ 6.25pm - 6.50pm
Part 5 - 17th February, 1979 @ 6.25pm - 6.50pm
Part 6 - 24th February, 1979 @ 6.30pm - 6.55pm

Notes:
*Featuring the Fourth Doctor, the First Romana, and K-9 MKII

*First appearance of the Black Guardian

*Part 6 of the 'Key to Time' story arc

*Working title: 'Armageddon'

*Part five was interrupted by a break in transmission; music played over a title card until the problem was fixed and the episode resumed