The Doctor and Romana had successfully retrieved five of the segments and now they have reached the planet Atrios in the middle of an atomic war, to search for the last, most vital piece.

Sinister dangers await them in this final stage of their quest...
 

The Doctor, Romana and K9 conclude their search for the six 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 the 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, recieving orders from behind a strange black mirror in the command centre.

The Doctor learns that no-one on Artios has ever seen a Zeon, and travels to Zeos in anattempt to bring the war to an end, and hopefully bring back Princess Astra - who seems 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. 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...

Transmission Date: 20th January 1979 to 24th February 1979

Running time: 146 mins approx.

Produced by Graham Williams
Directed by Michael Hayes