Los Alamos, 1944. A World War is being fought. In the American desert, the race is on to build an atomic bomb. The fate of the world is at stake, in more ways than one. The Seventh Doctor arrives, posing as a nuclear scientist; Ace is his driver and research assistant. They are here because someone, or something, is trying to alter the course of history at this most delicate point and destroy the human race. Playing detective among the A-bomb scientists, the Doctor tries to avoid falling under suspicion himself, but the head of Los Alamos security is convinced that something is not quite right about the small, eccentric Scottish research physicist calling himself Dr John Smith. As the minutes tick away to the world's first atom bomb test, the Doctor and Ace find themselves up to their necks in spies, aliens, and some very nasty saboteurs from another dimension.