Doctor Who and the Armageddon Factor — Terrance Dicks

Some time ago, the White Guardian, one of the most powerful beings in the Cosmos, had set the Doctor an urgent task - to find and reassemble the six segments of the Key to Time. -- 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...