Joseph Gastin is headmaster in the village school at La Rochelle. One of his pupils has supplied enough evidence to have him convicted of murdering the local postmistress. He persuades Maigret to go to La Rochelle and prove him innocent.

Chief Inspector Maigret returns to the town of his birth when he is passed an anonymous message warning him that a crime is to be committed in St Fiacre's Church on All Souls' Day.