Maigret goes undercover at a boarding house where a woman with two boys is occupying room number 4, hence Mrs. Quatre/Four. She has fled her home and husband with the boys and is in hiding after seeing a dead woman's body in the conservatory with her husband at night. Her reports to the police have no backing as the body is gone and her husband has also disappeared. Maigret takes on the investigation under the name Mr. Fountaine because a detail referring to a fountain, he believes, gives veracity to her story.

Maigret is called for by his nephew, a new inspector in the north after a man was found dead in a first class carriage. The 106 train to Paris which started outside France was stopped at the Jeumont station for 50 minutes during which the murder took place. His nephew had the carriage and dining car detached and isolated but ran foul of politics as a French Consol and his family, secretary, and nanny were on board. The victim's identity papers were stolen and the train is filled with a mixture of the guilty and the innocent and dentists. But of the murder? And who is and what happened to Max?

Maigret investigates the death of a seemingly boring accountant killed while standing at the bedroom window of his fourth floor flat. The victim had no strong feelings for his wife or his children, and a boring position at a bathroom fixture supplier, and has no friends, therefore, no enemies. His only love was a canary. Maigret calls his young nephew inspector to help find out just who the victim really was. He finds, literally, more of the same. But within that sameness, one difference.

Maigret investigates a threatening letter at the Parendon mansion.

A wealthy wine dealer's death doesn't bother his wife or "secretary."

A government minister asks Maigret to investigate when he becomes the victim of "dirty tricks".

Maigret is called to a farmhouse in Normandy; the owner has been killed. There he meets a 23-year-old woman named Felicie who is extremely defensive. She and Maigret keep up a barrage of unkind words the length of the story, it's wonderful to watch.

Maigret goes to a small village after receiving a letter from the son of the schoolteacher. The schoolteacher is currently in prison awaiting trial for the murder of the village's postmistress. She was supposedly killed when he was out of the classroom for a few minutes one morning collecting graded essays from his house which is attached to the school. The schoolteacher is an outsider in a very insular community. Maigret investigates a collection of drunks and children to find the truth.

A count who is also a diplomat is murdered.

Maigret is not at all pleased to become reacquainted with the dubious Leon Florentin, who was at school with him and now needs his help.

Dr. Baron is a respectable physician, married with two children. When his servant girl from Brittany dies of poisoning, her parents file a complaint with the police. Maigret travels to the doctor's home in Neuilly and investigates allegations of foul play associated with another servant; he also learns that a vagrant visited the house weekly.

While staking out a boarding-house where is staying Emile Paulus, a suspect, Detective Archambaud is wounded by a bullet shot from an undetermined place. To further the investigation, Maigret, his superior, decides to put up at the boarding-house posing as an ordinary guest. He is welcomed by Mademoiselle Clément, the dignified owner, and soon discovers the various people living in the place. He also takes a special interest in a woman who dwells in a neighboring block of flats, Françoise Boursicault, the lonely crippled wife of a purser most of the time at sea... A common thread through all of them is that... they seem to have something to hide...