In 1941 the German physicist Werner Heisenberg traveled to Copenhagen to see his Danish counterpart, Niels Bohr. These old colleagues revolutionized atomic physics in the 1920s, but they were on opposite sides in the second World War. When they met, the encounter ended in disaster. Recently revealed letters from Bohr suggest exactly why. Copenhagen fictionally recounts their exchange about the mysteries of the world and the even stranger mysteries of the world within.