Framed in the doorway of Poirot's bedroom stood an uninvited guest, coated from head to foot in dust. Who was he? Was he suffering from shock or just exhaustion? Above all, what was the significance of the figure 4, scribbled over and over again on a sheet of paper? Poirot finds himself plunged into a world of international intrigue, risking his life to uncover the truth about 'Number Four'.