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’. The story unfolds as Poirot, alongside his faithful assistant Hastings, follows clues and outmaneuvers a cabal of international criminals.

In this action-packed mystery, Poirot's brilliance is put to the test as he faces formidable foes, including a brilliant Chinese criminal mastermind, an American multi-millionaire, a beautiful Frenchwoman scientist, and "the destroyer," a ruthless murderer with a genius for disguise. Will Poirot succeed in foiling "The Big Four" and prevent their plan for world dominance?