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 K9 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.