Oliviero Rouvigny, a washed-up, alcoholic writer, lives in an old villa in Teolo, where he mistreats his wife Irina Rouvigny and their maid Brenda, hosting debauched parties for local hippies. Irina lives in fear of her husband and his cat Satan, which belonged to Oliviero’s late mother Esther, a former actress. Oliviero is oddly protective of the cat and of his mother's dress, the same gown she wore to portray Mary Stuart.
After his mistress Fausta is found murdered, Inspector Farla questions Oliviero when Fausta’s boss identifies him. Oliviero swears he was at home, and Irina reluctantly supports his alibi. That night, Brenda puts on Esther's dress, unaware of Oliviero’s intoxicated presence, and is attacked with a billhook—she dies as Irina discovers the body. Gripped by fear of being suspected in Fausta’s death, Oliviero walles up Brenda’s body in the cellar and orders Irina to discreetly clean his mother’s gown.
The next day, the Rouvignys pick up Oliviero’s visiting niece Floriana at a nearby train station, while a mysterious gray-haired man watches from a distance. That night, the gray-haired man delivers Esther’s dry-cleaned gown to Irina, whose indiscretion enrages Oliviero, prompting him to lock Irina in a closet to be clawed by Satan. Floriana teases her uncle about his attraction to her and hints at a rumored incest with Esther. She later frees Irina from the closet. Irina confides in Floriana about Oliviero’s abuse and cover-up, and the two share a sexual moment.
The killer murders prostitute Giovanna, and her madame kills the attacker, later revealed to be Bartello, who escaped a psychiatric hospital and assumed a new identity. Floriana begins dating motorbike racer Dario. After being forced to drop out of a race, Dario has sex with Floriana in an abandoned loft, where Oliviero spies on them. At the villa, local junk dealer Mrs. Molinar witnesses Irina stabbing one of Satan’s eyes out with scissors after Satan kills several of Irina’s pet doves.
That night, Oliviero discovers Floriana in Esther’s gown. She reveals she knew her uncle witnessed her and Dario’s tryst, and the two continue their affair. Floriana later informs Irina about the encounter and Oliviero’s growing urge to murder Irina, even suggesting that Irina pretend to die in an accident. Irina eavesdrops on Floriana’s and Oliviero’s pillow talk and overhears Oliviero’s plan to kill Irina and hide her body in the cellar. Oliviero finds Irina and beats her. Dario secretly invites Floriana to meet him in the morning to watch him race for a lucrative contract. Irina chases Satan into the cellar, where she uncovers Brenda’s corpse, convincing her that Oliviero is serious about his plan to murder her.
Irina ultimately kills Oliviero at his desk with scissors. Floriana reveals her desire for Esther’s valuable jewelry, which Irina then gives her after helping hide Oliviero’s body in the cellar. The two have sex. A figure enters the villa and repeatedly types “vendetta” on Oliviero’s typewriter. Floriana flees the villa. The figure is revealed to be the gray-haired man, Walter, Irina’s secret lover and partner in tormenting her husband. Irina had previously killed Esther and had Walter kill Brenda to cast police suspicion on Oliviero and drive him to doubt himself.
The next day, Walter creates an oil spill that causes the deaths of Dario and Floriana in a motorcycle accident. He retrieves Esther’s jewelry and returns it to Irina, who then pushes him off a cliff to his death. Irina returns to the villa, where Inspector Farla and a police officer arrive to investigate Mrs. Molinar’s report that Irina maimed Satan. They hear Satan’s meowing from the villa and trace the sound to the cellar, where they discover Satan walled beside Oliviero’s body, leaving Irina to confront the horror of what she has become.
