Belgium. It is sometimes forgotten, but the father of the world's most famous police detective is not French, but Belgian. Within six months, five lonely women are murdered in Montmartre. It's a challenge for Maigret: a conversation with a psychiatrist leads him to discover that he must first understand the killer's mental mechanism. In this episode of Maigret's adventures set in Montmartre, Simenon analyses with surgical precision a singular figure of the "serial killer", masterfully blending elements from psychoanalysis into his universe.