An unwanted visitor appears in Hercule Poirot's bedroom doorway, covered in dust from head to foot. The man pauses for a moment before swaying and collapsing. Who is he, exactly? Is he experiencing shock or simply exhaustion? Above all, what is the significance of the symbol 4, which has been drawn on a sheet of paper several times? Poirot is thrown into a realm of international intrigue, putting his life - and the lives of his twin brother - on the line to unearth the truth.