In Les Halles, in the heart of Paris, the long-divorced André Chatelin, an honest and respected man who runs a successful restaurant, is visited by an unknown young woman. She says she is Catherine, the daughter of his ex-wife Gabrielle, who has died leaving her homeless and penniless. André gives her a room and a job, but she soon starts abusing his kindness. She sows discord between André and his young friend Gérard, a medical student who is like a son to him. And she starts stealing money to support her mother, who is not dead but an ex-prostitute who is now a hopeless drug addict in a sordid hotel.
Feigning drunkenness one night, she gets André to help her to her bedroom and undress her, with the result that he marries her. As his heir she would be rich and free, so the next plan of her and her mother is to get rid of André. She decides to use Gérard, because of his medical knowledge, and swiftly seduces him. When he refuses to kill André, however, she strangles him and then pushes his car into the river with his corpse and his dog César. A grieving André identifies the body, leaving his telephone number with the police, who ring him to ask if he knows the woman seen in Gérard's car. From the description, it is obvious to him that it was Catherine. When he confronts her, she runs off to her mother's hotel. The dog César, who escaped drowning, follows her there and kills her.
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