:D In their flat one night, the conversation between Poirot and Hastings turns to the latter's belief that Poirot has never known failure in his professional career. The little Belgian tells him that is not the case and tells Hastings of one occasion when he did not succeed in unravelling a crime:
The event was the death of Paul Deroulard, a French Deputy who was living in Brussels. The time was the strife over the separation of church and state and M. Deroulard was a key player in these events as an anti-catholic and a potential minister. He was a widower, his rich young wife having died from a fall downstairs some years before. He inherited her house in Brussels and, although abstemious in terms of drinking and smoking, he had a reputation as a ladies man. He died suddenly in his house from reported heart failure on the eve of his promotion to minister of the state at a time when Poirot was a member of the Belgian detective force. He was taking a vacation when he received a visit from Mademoiselle Virginie Mesnard who was a cousin of M. Deroulard's dead wife who was convinced that the death was not natural. M. Deroulard's household consisted of four servants, his aged, but very infirm aristocratic mother, Mademoiselle Mesnard herself, and on the night of the death, two visitors: M. de Saint Alard, a neighbour, and John Wilson, an English friend.
Poirot was introduced into the household under a false pretext by Mademoiselle Mesnard and he began by investigating the meal served on the night of M. Deroulard's death but found no leads there. Looking in the study where the death actually occurred, Poirot spotted an open but full and untouched box of chocolates and found out that M. Deroulard ate some chocolates every night after dinner and finished the previous box on the night of his death. However, he noticed that the two boxes, one blue and one pink, had had their lids switched. Poirot then spoke to the dead man's doctor and discovered that M. de Saint Alard was an ardent Catholic whose friendship with M. Deroulard was being sorely strained by the political turbulence at the time. The doctor was also able to furnish examples of the types of poison that could be introduced into the chocolates which would have induced the type of death suffered. This caused Poirot to question local chemists where he found out that apart from eye drops for the aged Madame Deroulard, a prescription was made up for John Wilson of trinitrin within tiny tablets of chocolate (the medication being given to lower blood pressure). A large enough dose would prove fatal and could have been hidden in one of the chocolates.
This latest development caused a problem for Poirot as Wilson had the opportunity but not the motive whereas the position was reversed for M. de Saint Alard. Poirot then wondered why John Wilson had not come from England with enough of his medication to last him throughout his visit and he discovered from a maid in the house that a bottle of the tablets had been "lost". He decided to investigate the house of M. de Saint Alard in the Ardennes and, using the disguise of a plumber, he discovered in the bathroom cupboard there the empty bottle of medication. He returned to Brussels and it was then he obtained a summons from Madame Deroulard. Having discovered that Poirot was a police officer, she confessed to the murder of her son. Some years before she had seen him push his wife down the stairs and had realised the sort of man she had brought into the world. Afraid of the persecution that his new role would bring upon the church, she resolved to kill him. She took John Wilson's tablets and opened a new box of chocolates before seeing that one remained in the previous box. Into this she put the tablets and she put the empty bottle into M. de Saint Alard's pocket thinking that his valet would throw it away, not put it in the cupboard.
Madame Deroulard died a week later of her infirmities leaving Poirot to consider his mistakes: He knew Madame Deroulard had bad eyesight so no one else would have switched the lids on the two boxes of chocolates. Also, if M. de Saint Alard had been the criminal, he would never have kept the empty bottle. To this day, Poirot laments the failure of the little grey cells on that occasion - although, as Hastings notices, not enough to prevent him boasting of the other times when they have served him well!