While walking along a Paris street, Englishman Robert Stevens is shot by an unknown assailant, but luckily he is only struck glancingly and rendered unconscious. When he awakens in Beaujon Hospital, he initially thinks he was injured in an aeroplane crash. His father, Sir James Stevens, confirms he left England in an aeroplane, but ten days before. However, his father does not believe he cannot remember anything about those missing ten days. (It turns out that Robert is an irresponsible ne'er-do-well who had disappeared before.) Robert decides to find out what happened. His only clue is a note that was found on him signed by "D."