S05E06 - The Chocolate Box

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.