A mysterious "Le Furet" (the Ferret) sends letters to the Parisian Police telling them when murders are going to be perpetrated, and in which Paris district. Each time nothing is stolen, but money is somehow involved in inheritance,love blackmail,academic competition and jealousy,crime of passion,poison by woman,etc. After a street accident, they even catch a man with the letters ready to be posted but he claims to be only a clairvoyant. Each time the Police frames the wrong person and it's only after the sixth crime they seem to start to come out of their routine with the arrival of Mr.Wens who finds a corpse buried in the barn of a taxidermist.The other suspects are then released. The "professor Star", the clairvoyant sees in his crystal ball a crime in an oncoming train. The Police goes to the station but the woman is not dead.Then he sees a crazy woman set fire to a house and it's true. Star is released and all this has brought him fame, and fortune. Who is the killer? Will Mr.Wens find the killer?