Framed in the doorway of Poirot's bedroom stood an uninvited guest,coated from head to foot in dust.The man's gaunt face stared for a moment, then he swayed and fell.

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'.