In 1941 the German physicist Werner Heisenberg made a strange trip to Copenhagen to see his Danish counterpart, Niels Bohr. They were old friends and close colleagues, and they had revolutionised atomic physics in the 1920s with their work together on quantum mechanics and the uncertainty principle. But now the world had changed, and the two men were on opposite sides in a world war. The meeting was fraught with danger and embarrassment, and ended in disaster.