A dazzling display of order, method, and Poirot's "little grey cells" . . .
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 four, scribbled over and over again on a sheet of paper?
In this mystery, Poirot finds himself plunged into a world of international intrigue, risking his life to uncover the significance of "Number Four".