Déroulard's household consists of four servants, his aged and infirm aristocratic mother, herself, and on that night, two visitors: M. de Saint Alard, a neighbour from France, and John Wilson, an English friend. Virginie introduces Poirot into the household and he begins interviewing the servants about the meal served on the night of M. Déroulard's death. He suspects poison, but all ate from common serving dishes. In the study where the death occurred, Poirot spots an open but full and untouched box of chocolates. M. Déroulard ate some chocolates every night after dinner, finishing a box on the night of his death. The servant retrieves the empty box. Poirot notices that the lids of the two boxes, one blue and one pink, are switched. As only Déroulard ate the chocolates, the new box should not have been opened yet. His doctor says Saint Alard and Déroulard had argued strenuously that evening, justifying the cause of death. Poirot finds the local chemist who made eye drops for the aged Madame Déroulard, for her cataracts. He finds the "English chemist" who had made up the prescription of trinitrin for John Wilson, small tablets in chocolate to lower blood pressure. Too many tablets at once would prove fatal. This development bothers Poirot. Wilson had the opportunity but not the motive whereas the position is reversed for M. de Saint Alard. Virginie gives Poirot the address of Saint Alard in the Ardennes. In the disguise of a plumber, Poirot breaks into the house and discovers an empty pill bottle from Wilson in the bathroom cupboard. Poirot thinks he has solved the case. In Brussels, Madame Déroulard summons Poirot. Learning that Poirot was a police officer who had concluded his investigations, she confesses to the murder of her son. Some years before, she saw him push his wife down the stairs and realised that he "was an evil man". Afraid of the persecution that her son's new role would bring upon the church and for his approaches towards the innocent Virginie, she resolved to kill her son. She stole John Wilson's tablets. She opened the new box of chocolates before seeing that the previous box was not yet empty. After inserting about 20 tiny tablets in one chocolate, she took the opportunity to place the empty bottle into M. de Saint Alard's pocket when he came to say his farewell, thinking that his valet would