The Armageddon Factor

From: http://www.bbc.co.uk/doctorwho/classic/episodeguide

The final segment of the Key is traced to the planet Atrios, engaged in a long war with the neighbouring Zeos. The Marshal of Atrios intends a final strike to destroy the Zeons, but the Doctor and Romana discover that Zeos is deserted and the war is being co-ordinated by a computer called Mentalis, built by one of the Doctor's old Time Lord Academy friends, Drax.

The computer is under the control of the Shadow, an agent of the Black Guardian. He and his servants, the Mutes, are inhabiting an unseen third planet positioned between Atrios and Zeos.

The Doctor creates a temporary substitute for the final segment from a substance called chronodyne and uses the Key to place a time loop around the ship from which the Marshal is about to launch his strike against Zeos. It transpires that Atrios's Princess Astra is the real sixth segment. The Shadow converts her into the segment, but the Doctor snatches it and escapes to the TARDIS, where he finally completes the Key.

The White Guardian appears on the scanner screen and congratulates the Doctor. He asks that the Key be given over to him, but the Doctor decides that it is too powerful for any one being to control and orders it to re-disperse.

Enraged, the Guardian reverts to his true colour - Black - and vows that the Doctor shall die for his defiance. In order to shake him off, the Doctor fits a randomiser to the TARDIS's controls. There is now no telling where or when his travels will take him.

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