'Atom Bomb Blues' by Andrew Cartmel

Los Alamos, New Mexico, 1945. The Second World War is coming to its bloody conclusion, and 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.

Someone, or something, is trying to alter the course of history at this most delicate point. And destroy the human race. Posing as a nuclear scientist with Ace as his research assistant, the Doctor plays detective among the Manhattan Project scientists, while desperately trying to avoid falling under suspicion himself.

As the minutes tick away to the world's first atom bomb blast, the Doctor and Ace find themselves up to their necks in spies, aliens of the flying saucer variety, and some very nasty saboteurs from another dimension.

Notes:
*Featuring the Seventh Doctor and Ace

*A BBC Books 'Past Doctor Adventures' novel

*Time-placing: Ace refers to Doctor Judson, implying that this takes place just after 'The Curse of Fenric'