Hungarian physicist Leo Szilard leaves Europe, eventually arriving in the United States. With the help of Einstein, he persuades the government to build an atomic bomb. The project is given to no-nonsense Gen. Leslie Groves who selects physicist J. Robert Oppenheimer to head the Los Alamos Laboratory in New Mexico, where the bomb is built. As World War II draws to a close, Szilard has second thoughts about atomic weapons, and policy makers debate how and when to use the bomb.

Brian Dennehyas Gen. Leslie Groves
David Strathairnas J. Robert Oppenheimer
Michael Tuckeras Leo Szilard
Hume Cronynas James F. Byrnes
Richard Dysartas President Harry S. Truman
Hal Holbrookas Gen. George Marshall
Barnard Hughesas Henry Stimson
John McMartinas Dr. Arthur Compton
David Ogden Stiersas Franklin D. Roosevelt
Anne Twomeyas Kitty Oppenheimer