Susan, a newly married young woman still wearing her bridal gown, leaves on honeymoon with her new husband and eventually arrives at a hotel. Another woman seems to be stalking the couple from her position in a nearby car, and when Susan is left alone in the room for just a few moments, she has a violent fantasy of a strange man leaping out of the closet and raping her. When her husband returns she insists on leaving, and so they do.
The couple arrives at a house where the husband apparently grew up. This is where the rest of the story unfolds. The wife sees the woman from the hotel in the woods on the property but she does not tell her husband. Susan notices in the house that there are paintings up of male ancestors but none of the wives. She is told by the servants' daughter that the wives' paintings are in the cellar. Susan notices that one of the paintings of the wives has the face cut out of it. Susan's husband tells her that the woman in the painting is named Mircalla Karnstein, one of his ancestors, who two hundred years before murdered her husband on their wedding night because he supposedly made her commit unspeakable acts. Susan has violent dreams involving the mysterious woman she has been seeing. She wakes up and finds a dagger under her pillow. At his point, Susan starts to become detached from her husband. The husband calls on a doctor to figure out why she is having all of these dreams and what is wrong with her. Soon Mircalla is invading Susan's dreams, persuading her to use a mysterious dagger, which keeps reappearing no matter where it is hidden, to butcher Susan's husband as Mircalla did hers.
One day while strolling out on the beach, the husband discovers a naked woman buried in the sand, with only her snorkel providing air. He digs her out and takes her home where she reveals herself to be Carmilla. Susan falls under the spell of Carmilla, a man-hating vampire who seduces Susan and puts the bite on her to drink her blood and possess her in a lesbian relationship. The husband finally catches on that Carmilla is really his ancestor Mircalla Karnstein and a vampire, and that his life is in danger. The repressed Susan's desires are awakened in the intense lesbian love affair and she embarks on a spree of bloody mayhem. They kill the doctor, the guardian of the property, and they try to kill the husband too, but he kills them while the two women are resting in their coffins as vampires. (Oddly enough, in a departure from the traditional methods to kill a vampire, he first uses a rifle to shoot the coffin multiple times.) After this happens, the servant's daughter arrives, and reveals that she was bitten too; she then kneels and allows the husband to shoot her once in the head. He returns to the coffin with a dagger, and the scene cuts to a newspaper column declaring, "MAN CUTS OUT THE HEARTS OF THREE WOMEN," suggesting the husband was found and arrested for three murders.