Assigned to oversee the development of the atomic bomb, Gen. Leslie Groves is a stern military man determined to have the project go according to plan. He selects J. Robert Oppenheimer as the key scientist on the top-secret operation, but the two men clash fiercely on a number of issues. Despite their frequent conflicts, Groves and Oppenheimer ultimately push ahead with two bomb designs — the bigger "Fat Man" and the more streamlined "Little Boy."