Maigret goes to Saint-Fiacre, his home town, after noticing a note that had been received by the Moulins police, and sent up to Paris, warning that a crime was going to be committed in the Church at Saint-Fiacre, during the first Mass. And in fact, before Maigret's very eyes, the old Countess de Saint-Fiacre dies during the mass... of heart failure. Maigret locates her missal, in which a fake newspaper story of her son's suicide had been planted. Her son, Maurice de Saint-Fiacre appears with his mistress, Marie Vassilief. He is broke, as usual, had come to borrow more from his mother. His mother's secretary and lover, Jean Métayer is quick to protest his innocence. The steward, Gautier, who lives in the house that had been Maigrets's father's, tells Maigret that the Countess had been going broke as well, and that he had been paying many of her debts. His son, Émile Gautier, works in the bank in Moulins, where Maigret verifies the state of the estate.
Maurice schedules a dinner, at which all concerned are invited, including the priest and Dr. Bouchardon, and Jean's lawyer. During the dinner Maurice promises that the murderer will be dead by midnight, and at midnight Émile shoots him, and then explains that he'd known he'd done it. But the gun contained blanks, and Maurice is quick to show that it was the steward and his son who had done the deed, which, however, was unpunishable as a legal crime.
This report prepared by Dana Samson