In 1941 the German physicist Werner Heisenberg (Daniel Craig) made a strange trip to Copenhagen to see his Danish counterpart Niels Bohr (Stephen Rea). They were old friends and close colleagues and they had revolutionised atomic physics in the 1920's 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. Why Heisenberg went to Copenhagen and what he wanted to say to Bohr are questions which have excercised the minds of historians ever since. In Michael Frayn's play, Heisenberg meets Bohr and his wife Margrethe once again to look for the answers and to work out, just as they had once worked out the internal functioning of the atom, how we can ever know why we do what we do.