The Armageddon Factor

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 the Shadow's power. Drax again uses the stabiliser, this time to return them to their normal size. The Doctor snatches the partially assembled Key and the final segment, and disappears with Romana and K9 in the TARDIS, assembling the Key after setting up a shield around Zeos to deflect the Marshal's missiles towards the Shadow's base, destroying it.

The White Guardian appears to congratulate the Doctor on finding and assembling the Key to Time, and requests that it be sent to him. However, the Doctor, realising that it is actually the Black Guardian in disguise, orders the Key to re-disperse. Enraged, the Black Guardian threatens to kill him. In an attempt to evade him, the Doctor fits a randomiser into the TARDIS guidance system, sending it to an unknown location in time and space, leaving the Doctor with no idea of where they are headed, and the Guardian being unable to follow.

Paul Cornell, Martin Day, and Keith Topping gave the serial an unfavourable review in The Discontinuity Guide (1995), describing it as "a dreary end-of-season Oh-my-God-the-money's-run-out 'spectacular'" without subtle acting.[2] In The Television Companion (1998), David J. Howe and Stephen James Walker wrote that The Armageddon Factor was "entertaining enough in itself, with some good direction by Michael Hayes and generally fine production values, but ultimately fails to tie up all the loose ends and leaves the over-arching plot strangely unresolved". They praised the Shadow but felt that the other characters were one-dimensional, and called the ending a "cop-out".[3] In 2011, Patrick Mulkern of Radio Times stated that the serial "hugely disappoints, yet it's not an unmitigated disaster". He criticised the characterisation and much of the plot, but praised the direction and the Shadow.[4] On the other hand, DVD Talk's Justin Felix gave the serial four out of five stars, saying that it "packs more of a wallop than the previous two stories" and had everything typical of Doctor Who. Felix also called it Mary Tamm's best performance.[5]

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• The Armageddon Factor at BBC Online