In Roman Polanski's second feature, and first English-language film, he did the unthinkable. He took Catherine Deneuve, the international poster child for goodness, frailty, and beauty, and made her into a sex-obsessed killer. He achieved international notoriety, and produced a fascinating if slow-moving study of the onset of madness.

Carol is a Belgian girl living in England with her sister Helen (Yvonne Furneaux). Carol detests Michael (Ian Hendry), her sister's boyfriend, who is a lecher and a jerk. She works at a beauty parlor, where her job is to stare off into space. She is strikingly beautiful and painfully meek, and when her boyfriend Colin pressures her too hard, she retreats to the comfort of her apartment. She is both obsessed with and repulsed by sex; listening to Helen and Michael in the next room, but furious when she finds a stained garment.

Her break with sanity occurs when Helen and Michael go on vacation. She wanders the apartment for several days, not going to work. From her subjective viewpoint, the apartment becomes a house of horrors. Rooms change dimensions, walls threaten to split in two while phantom hands emerge to grab her. She is accosted by several leering men who rape her in her imagination. When the worried Colin (John Fraser) breaks in to check on her, she bludgeons him to death with a candle holder and stores him in the bathtub. As her sanity inexorably disappears, she is confronted by more demons both within her mind and in the apartment.