Dapper Inspector Vorobechik ('Wens' for short) is assigned the case of a serial killer who leaves a calling card on his victims; Monsieur Durand. Wens' mistress, struggling actress Mila Malou, determines to get publicity for herself by helping him. Learning that Durand is one of the eccentric tenants of a boarding house at No. 21 Avenue Junot, Wens takes a room in the guise of a Protestant minister; only to be followed by Mila who hardly seems like a minister's wife! Suspects are arrested, but while each is in jail, there's another murder...