20 January 1979
John Woodvine (Marshal) [1-4,6], Lalla Ward (Princess Astra), Davyd Harries (Shapp) [1-4,6], Ian Saynor (Merak) [1-4], Ian Liston (‘Hero‘) , Susan Skipper (‘Heroine‘) , John Cannon , Harry Fielder  (Guards), William Squire (The Shadow) [3-6], Iain Armstrong (Technician) , Pat Gorman (Pilot) [3-4]*, Barry Jackson (Drax) [5-6], Valentine Dyall (The Guardian) .
|Written by||Bob Baker And Dave Martin|
|Directed by||Michael Hayes|
|Produced by||Graham Williams|
The Doctor, Romana and K-9 conclude their search for the segments that make up the powerful Key to Time. Arriving on the planet Atrios, the Doctor and his companions become embroiled in the Atrians’ seemingly endless war with their neighbouring planet of Zeos. Their ruler, the Princess Astra, has been kidnapped, leaving the deranged Marshal in sole charge of the war effort. However, the Marshal is being controlled, receiving orders from behind a strange black mirror in the command centre. The Doctor learns that no-one on Atrios has ever seen a Zeon, and travels to Zeos in an attempt to bring the war to an end, and hopefully find the Princess Astra – who seems to be linked in some way to the Sixth Segment. He finds the planet deserted, run entirely by a huge computer, Mentalis.
When the Marshal launches an all-out final attack on Zeos, Mentalis responds by activating its Armageddon Program – rather than lose the war, the computer will destroy everything. However, a third planet hangs in space between Atrios and Zeos, shielded from their sensors. This is the lair of the Shadow. He has been waiting for the Doctor to arrive, manipulating the war to his advantage. He is willing to use the Doctor’s own friends against him and he wants the Key to Time. Although the Doctor’s quest is nearly over, he begins to realise that he may lose everything. For the Shadow is a servant of the Black Guardian.
- The Black Guardian reappears in The Well-Mannered War, Mawdryn Undead, Terminus, and Enlightenment, which also features the White Guardian.
- The Randomizer installed here remains in the TARDIS until the Doctor uses parts from it to reconfigure the generator in The Leisure Hive.
- Drax appears again in the flashback scene in Divided Loyalties, and in an alternative timeline in The Quantum Archangel.
- Drax mentions the Doctor’s nickname At The Time Lord Academy, Theta Sigma, which is mentioned again in the flashback scene in Divided Loyalties, and briefly in The Happiness Patrol, when the Seventh Doctor tells Trevor Sigma that his nickname at school was Theta Sigma.
- It’s been suggested that The torture Romana undergoes At The Shadow’s hands may be responsible for her subsequent regeneration in Destiny of the Daleks, in the audio Gallifrey: Lies, this is confirmed in a way, although the connection between the two is somewhat more complicated than expected.
- Mary Tamms last story and Lalla Wards first appearance on the show, replacing Tamm from Destiny of the Daleks
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