THE ARMAGEDDON FACTOR by Bob Baker & Dave Martin
Story 103

Synopsis:
The final segment seems to be located on Atrios.
En route, the Doctor is puzzled that the Locator is giving unusual readings. Down on Atrios, a nuclear war is going badly. Zeos seems to have the upper hand, and the Marshal is determined not to let anyone deter him from his course. The Doctor and Romana meet a surgeon, Merak, who is concerned about Princess Astra. The Marshal seems to change his mind, demanding that the Doctor help him win the war. Confused, the Doctor deduces there must be a barrier surrounding Zeos, and that someone is manipulating the Marshal. He is allowed to visit Zeos, but is met by the Shadow, who demands the Key To Time. The Doctor refuses, and the Shadow predicts he will fail anyway. Romana and Merak reach Zeos, looking for Astra. The Marshal leads an attack on Zeos. The Doctor discovers that the Zeon operation is run by Mentalis, a computer, which is programmed to destroy both planets in the event of attack. To prevent this, the Doctor creates a fake 6th segment, using the Key to put Atrios & Zeos into a time loop. Meanwhile, the Shadow controls Astra and K9, and has Merak as a prisoner. The Doctor and Romana arrive on the Shadow's planet. Separated, Romana is taken prisoner by the Shadow. The Doctor meets Drax, a Gallifreyan who built Mentalis for the Shadow. Together, they take control of K9, and plot a way into the Shadow's lair. Astra is revealed to be the elusive 6th segment. As the Key is completed, the Doctor arrives and steals it back. Drax deals with Mentalis, whilst the Marshal's attack is redirected to destroy the Shadow's planet. The White Guardian appears on the TARDIS scanner, demanding the Key to Time. Deducing that this is a ruse, the Doctor thwarts the Black Guardian, casting the Key asunder once more, restoring Astra. He then fits a Randomiser to the TARDIS so the Black Guardian cannot catch him.
Review:-
The quest for the Key To Time draws to a conclusion with the highest stakes on the table, and a threat of oblivion facing everyone.
Atrios and Zeos may seem like two easy opposites, and they are. That the story starts from Atrios' point of view is cunning, because we soon see how bad the situation is, and the viewers can make judgements about the Zeons, too. There are clues that all is not as it seems, and hints of a 3rd party, but it is only when the Doctor goes to Zeos that more of the picture is seen. The total lack of Zeons is fine, because Mentalis covers for this, and by this stage, the Shadow has made himself plain. With the stakes upped, the Doctor is forced to pull quite a fast one in time looping the Marshal, and then start hunting the Shadow, and the elusive 6th segment...
The nuts and bolts of the story are all well enough, but is it any good? By and large, keeping it simple, yes it is. Rather like the later
The Caves Of Androzani, the stakes are accentuated by the fact that everyone has an ulterior motive, and no-one wants to be considerate. Even non-villain characters like Merak and Drax are less than helpful at times. With safety taken away from them, the Doctor and Romana have to risk their lives over and over to stop the Marshal, Mentalis and the Shadow, all the while trying to find why Princess Astra is linked to the missing 6th segment.
The decision to make the final segment a living person provides an added dilemma to the end-of-epic drama, as the Doctor tries to dodge the wrath of the Black Guardian. There is little question for the Doctor that it would be morally wrong to deny Astra life so that a mere symbol like the Key To Time could exist for a long period of time. His actions allow Merak and Astra their happy ending, even at the cost to his own liberty. Such in a nutshell, is the long-standing magic of the series.

As a straightforward story that builds to a welcome conclusion, I can recommend this.
Disclaimer: I've read the book, and seen the video.
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