On holiday, Maigret would like to show his wife the places of his childhood, the small village of St. Fiacre, to which a castle belongs. During the Sunday service, the two witness an unpleasant incident: the Countess suddenly collapses dead. Although the doctor diagnoses a heart attack as the cause of death, Maigret becomes suspicious because the Countess's prayer book is untraceable. During a visit to the castle, the inspector discovers that there are quite a few people who wished the old countess dead ... Based on the 13th Maigret novel L'Affaire Saint-Fiacre (1932), published in German under the title Maigret und das Geheimnis im Schloß or in a later translation as Maigret und die Affaire Saint-Fiacre.