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 travels to Dole after a gun found with a murdered "vagrant" matched one used to kill another "vagrant" under a Paris bridge. There he encounters a pair of feuding sisters, one of whom owns the shed the victim was found in. The feud has been going on for 30 years, the source of which is both clear and muddied at the same time, just like the weather.