While Hercule Poirot is holidaying in France, a businessman tells Poirot that his life is in danger. Poirot sets out to investigate, but he arrives too late to save his client, whose brutally stabbed body now lies face downwards in a shallow grave on a golf course.
But why is the dead man wearing an ill-fitting coat with a mysterious love letter in its pocket? And is the secretive neighbour a blackmailer or mistress? Before Poirot can answer these questions, the case is turned upside down by the discovery of a second, identically murdered corpse.
While the local authorities pursue the false leads suggested by the evidence, Poirot relies instead upon his famous “little grey cells” to cut through the confusion and untangle a story of blackmail, forbidden love, and a long-buried secret.