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.