Inspector Wenceslas (referred to as "Wens" for short) Vorobeychik is assigned the case of a serial killer who leaves a calling card with the name 'Monsieur Durand' on his victims. Wens' mistress is the ditsy struggling actress Mila Malou who is determined to get noticed, and attempts to help Wens find the criminal. Wens discovers that Durand is a tenant at a boarding house at No. 21 Avenue Junot. Wens takes a room at the house in disguise as a Protestant minister. Suspects are arrested, but while each is in jail another Durand murder occurs. Both Mila and Wens discover who is responsible for the murders. Wens is captured, and as he is about to be killed when Mila and the police arrive and rescue him.