Parts: 6
Writers: Bob Baker, Dave Martin
Airdates: 20 January 1979 - 24 February 1979
Production Code: 5F

Doctor: fourth
Companion: the first Romana, K-9 mark two

Megalomaniacs of the Week: the Shadow, the Black Guardian

Story: In search of the sixth and most important segment of the Key to Time, the Doctor, Romana and K-9 travel to the planet Atrios - but their visit is sadly ill-timed, as Atrios is in the midst of a vast nuclear war with its twin planet, Zeos. The Marshal of Atrios and the planet's Princess, Astra, are at odds over the war. The Marshal is enthusiastic and determinedly single-minded about fighting, while Astra is opposed to the war and equally determined to end it, if possible.

The Marshal is behaving unusually, talking to his reflection and seeming to take his orders from an unseen power. When he has Astra imprisoned in a radioactive part of the planet, the princess is abducted by a mysterious figure in black. The Doctor manages to get on the Marshal's good side, and decides to go to Zeos via transmat in order to develop a psychological weapon that will protect Atrios. Romana, meanwhile, has found a way to get behind the Marshal's mirror - and realizes that he is being given orders by a third power...someone who wants to capture the Doctor. But before she can inform the Doctor that he is walking into a trap, he has left for Zeos - accompanied by two more of the mysterious robed men.

The Doctor finds himself in the company of the Shadow, a sinister being who has captured Princess Astra and wants control of the Key to Time. As the war between the Atrions and the Zeons escalates, the Doctor realizes that the Shadow is pulling the strings. As events come to a head, the Doctor can only delay the inevitable to keep both Atrios and Zeos from being utterly destroyed. And if the Shadow has his way, even this will pale in comparison...for if the Black Guardian gets control of the Key to Time, the universe will be plunged into darkness for eternity.