Atomic scientists under military supervision work on the secret Manhattan Project to invent and use the first atomic bombs.
A newsreel shows the burying of a time capsule by atomic scientists. Robert Oppenheimer (Hume Cronyn) introduces the film, which begins with German efforts to develop weapons. Matt Cochran (Tom Drake) shows Enrico Fermi (Joseph Calleia) and Dr. Marre the splitting of an atom. Fermi says he will work on a bomb. Leo Szilard (John Gallaudet) tells Matt that a chain reaction is possible, and the experiment works. They go to Albert Einstein (Ludwig Stossel), who says millions could be killed. Einstein writes to President Roosevelt. Germany attacks countries.
Van Bush (Jonathan Hale) gives a report to Franklin Roosevelt (Godfrey Tearle), who calls Churchill. British scientists arrive. Matt explains to Major Jeff Nixon (Robert Walker) how the uranium atoms cause a chain reaction. In the Chicago lab they make some plutonium. Some scientists resign because they will not work on munitions. Jeff reports to General Leslie Groves (Brian Donlevy), who is put in charge of the secret atomic bomb project. Corporate leaders check into a meeting and are warned about the Espionage Act. Uranium plants are built at Oak Ridge, Tennessee. Groves orders Jeff and Matt to relocate 18,000 people from Hanford, Washington. They return to Los Alamos. Matt and his wife Ann (Beverly Tyler) move into a bungalow. Oppenheimer asks Groves for 200 precision toolmakers. Groves learns they need six million pounds of copper, and he says they can use silver from the US Treasury.
FDR wants to stop all wars and dies. Groves, Oppenheimer, General Thomas Farrell (Henry O'Neill), Jeff, and Matt prepare for the first test in the desert. Farrell asks about a chain reaction in the air, and Oppenheimer says the chance is less than one in a million. The bomb explodes and forms a mushroom cloud. President Truman in Potsdam says he gave Japan an ultimatum that threatened prompt destruction, and he hopes the bomb will shorten the war.
Matt questions if it is right to kill so many. Matt and Jeff prepare the bomb in the Mariana Islands. Col. Paul Tibbetts (Barry Nelson) briefs the air crews and shows a film of the test. While assembling the bomb, Matt burns his hand and knows he will die soon. The plane takes off, and Jeff gets the bomb ready. They fly over Hiroshima at 8:15 a.m. and drop the bomb. They see the explosion and the cloud. They go back and see the fires raging. Farrell and Groves get the message that it worked.
Jeff arrives in Washington on a plane and tells Ann that Matt died. They read Matt's letter that balances the fear of atomic war with the hope that atomic energy could be used for good.
Other than the fictional character of Matt, this seems
to be a rather accurate dramatization of the massive project that
led to the historic breakthrough which threatens the future of
humanity. Whether humans will learn to get along or wreak mutual
destruction on each other, just as early humans learned how to
use fire, is still an open question.