Focusing on a scientific mystery that has intrigued and baffled historians for more than 50 years, this documentary relates the story behind a 1941 meeting between two brilliant physicists, Niels Bohr of Denmark and Germany's Werner Heisenberg. Long-time friends whose work together had opened the way to the atomic bomb, they ended up on opposite sides of World War II. Heisenberg made a covert trip at great risk to see Bohr and his wife, Margrethe, in Copenhagen, but the meeting ended in disaster. Why did Heisenberg go to Denmark, and what did the two men say to each other? What happened during this pivotal meeting-a defining moment of the modern nuclear age?