The Chocolate Box

Hercule Poirot relates the one occasion when he failed to solve a crime through an error in his own reasoning, years earlier when he was a police detective in Brussels.

Paul Déroulard, a French Deputy who was living in Brussels, had died from heart failure. Mademoiselle Virginie Mesnard, a cousin of his late wife, asks Poirot to investigate.

In the study where the death occurred, Poirot spots an open but full and untouched box of chocolates. Paul ate some chocolates every night after dinner, finishing a box on the night of his death. Poirot requests that a servant bring the empty box and notices that the two lids, one blue and one pink, have been switched.

Poirot learns that John Wilson, a friend of Paul who had visited him that night, had recently had a prescription filled for tablets of trinitrin (nitroglycerin) in chocolate. Although Wilson had the means to kill Paul by stuffing these tablets into his chocolates, Poirot cannot discern a motive. He discovers that Wilson had needed the tablets because the ones he had brought from home had gone missing, and later finds an empty bottle at the home of M. de Saint Alard, a visitor with whom Paul had had a heated argument.

Poirot concludes that M. de Saint Alard used Wilson's tablets to poison Paul; when he reports to Paul's mother, though, she confesses to the crime herself. She had seen her son push his wife down the stairs to her death years earlier, but knew that her poor eyesight would make her an unreliable witness at trial. To avenge the death, she stole the tablets, poisoned the last chocolate in Paul's box, and slipped the bottle into M. de Saint Alard's pocket as he left, expecting that his valet would find it and throw it out. However, she first opened the full box by mistake, then switched the lids because she could not see the different colors. Poirot closes his investigation, and Madame Déroulard dies a week later of her infirmities.