When Hungarian physicist Leo Szilard leaves Europe he eventually arrives in the United States where, with the help of Albert Einstein, he persuades the Federal government to build an atomic bomb. General Leslie Groves 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 (whose idea was responsible for the progress made) has second thoughts about atomic weapons and debates how and when to use the bomb.