This is a very special case in Maigret's experience, in which he invests his heart as much as his ingenuity. A nice old lady, meticulously groomed and showing no signs of derangement, timidly tries to see the famous detective and finally accosts him in the street. She is frightened: someone invades her apartment during her absences. Nothing is missing. But, says she, there are minute changes in the positions of objects, which to her prove the presence of an intruder. Maigret's subordinates shrug her off as a lunatic, and she becomes known at Police Headquaters as Maigret's Madwoman. But Maigret is touched by the look in her eyes and promises to go and see her. Someone else, however, gets there ahead of him.

This sets the stage for a hunt that takes Maigret into the underworld of the Riviera and brings an innocent woman close to disaster. Madame Maigret, for once, modestly interferes and helps to throw light on the case by her understanding of female psychology. Maigret is shrewd enough to see her point.