Hercule Poirot is preparing for a voyage to South America. Looming in the doorway of his bedroom is an uninvited guest, coated from head to foot in dust and mud. The man's gaunt face registers Poirot for a moment, and then he collapses. The stranger recovers long enough to identify Poirot by name and madly and repeatedly scribble the figure '4' on a piece of paper. Poirot cancels his trip. An investigation is in order. Fortunately, Poirot has the faithful Captain Hastings at his side as he plunges into a conspiracy of international scope — one that would consolidate power in the deadly cabal known as 'The Big Four.'