Some time ago the White Guardian, one of the most powerful beings in the Cosmos, set the Doctor an urgent task to find and reassemble the six segments of the Key to Time. Having successfully retrieved five of the segments, the Doctor and Romana arrive on the planet Atrios in the middle of an atomic war. There they must search for the last, most vital piece of the Key. However, sinister dangers await them in this final stage of their quest. The actions of the Marshal of Atrios, operating under a mysterious influence, look certain to bring about armageddon for them all.