Belgian detective Hercule Poirot is summoned to France after receiving a distressing letter with an urgent cry for help. Upon his arrival in Merlinville-sur-Mer, the investigator finds the man who penned the letter, the South American millionaire Monsieur Renauld, stabbed to death and his body flung into a freshly-dug, open grave on the golf course adjoining the property. Meanwhile the millionaire's wife is found bound and gagged in her room. Apparently, it seems that Renauld and his wife were victims of a failed break-in, resulting in Renauld's kidnapping and death.
There's no lack of suspects: his wife, whose dagger served as the weapon, his embittered son, who would have killed for independence, and his mistress who refused to be ignored. Each felt deserving of the dead man's fortune. The police think they've found the culprit. But Poirot has his doubts. Why is the dead man wearing an overcoat that is too big for him? And who was the impassioned love-letter in the coat pocket for? Before Poirot can answer these questions, the case is turned upside down by the discovery of a second, identically-murdered corpse.
