Maigret has scruples

Psychological finesse is required: Maigret is to investigate a case that isn't even a case yet. The Quai des Orfèvres is quiet until a visitor disrupts the general lethargy: Xavier Marton, the king of model train sales at the Grands Magasins du Louvre, accuses his wife of wanting to poison him. Hardly has he appeared when Marton disappears again. Maigret is ready to close the case. However, later that same day, Mrs. Marton comes to see him and claims that her husband is insane and is trying to kill her. Hesitantly, Maigret orders two of his officers to keep an eye on the couple. Then a third person enters the scene, Marton's sister-in-law, who seems to be very close to him. Who is really insane here, and who wants to kill whom? Maigret's 52nd case takes place in the 14th arrondissement.