Austria, 1830. A barmaid walking through the woods accepts a lift from a passing carriage. She is taken against her will to Castle Karnstein, where she is brought before Count Karnstein. He cuts the girl's throat and feeds the blood to a desiccated corpse. Within moments, the dust and bones are restored into a young blonde woman.

Horror writer Richard Lestrange is travelling through Europe researching his new novel. At an inn, the landlord tells him that every forty years the village is terrorised by the vampires of the Karnstein family. Lestrange scoffs, declaring that he will visit Castle Karnstein to prove that vampires don't exist.

Arriving at Castle Karnstein, Lestrange is startled by the approach of three hooded women. They are not vampires, however, but part of a history field trip from the nearby finishing school. Giles Barton, the girls' teacher, invites him back to the school.

Lestrange is introduced to the snobbish school principal, Miss Simpson. They are interrupted by the arrival of a new pupil, a young woman named Mircalla, accompanied by her mother, Countess Herritzen. Entranced by Mircalla's beauty, Lestrange offers his services to Miss Simpson as a teacher. She declines, saying that an English teacher is already on his way. Lestrange is undeterred.

Janet Playfair, a junior teacher, takes Mircalla for a tour of the school. She is introduced to her roommate Susan. That night, Susan and Mircalla kiss. Susan suggests they go for a midnight swim at the nearby lake. As Susan waits at the lake, another barmaid is drained of blood. Mircalla arrives and makes vague excuses for her lateness. They have sex, while Count Karnstein watches from the woods.

Lestrange intercepts the new English teacher at the tavern, tricks him into going to Vienna and steals his job. Lestrange shares lodgings at the school with Giles, who has also become obsessed by Mircalla, but for very different reasons: through his research he has become convinced that she is a Karnstein vampire.

That night Susan and Mircalla return to the lake. Mircalla drains her of her blood. Hidden, Giles sees what has happened. He throws Susan's body down a well to hide the evidence.

Fearing scandal, Miss Simpson covers up Susan's disappearance, but agrees to call the police if the girl doesn't reappear in the next forty-eight hours.

Giles arranges to meet Mircalla at midnight. He professes his love to her, and begs to become her servant. Mircalla responds by killing him. Giles's bloodless body is discovered the next morning. Countess Herritzen's personal physician announces that he died from a heart attack. Janet tells Lestrange that Susan is missing, and that Mircalla may be involved.

Examining Giles's papers, Lestrange discovers that Mircalla may really be the vampire Carmilla Karnstein. Lestrange confronts her with this information. Mircalla admits that she is a Karnstein, but has taken the name Herritzen to avoid persecution. Richard tells her that he loves her, and they have sex.

Countess Herritzen convinces Miss Simpson to allow her physician to declare Susan dead. Suspecting foul play, Janet calls in the local constable. He finds Susan's body in the well, but falls to his death when somebody cuts his climbing rope. Meanwhile, Mircalla hypnotises Janet and tries to bite her, but is driven away by the crucifix around her neck.

Susan's father arrives from England. He demands that his daughter is disinterred and the cause of death verified. Susan's body has been placed in the grave overnight. His doctor notes the bite marks on the girl's throat. Countess Herritzen tries to convince them that Susan killed herself, but they will not believe her.

A travelling priest stops at the local inn. Learning of the recent deaths, he raises a mob to destroy the vampires. Mircalla, Countess Herritzen and Count Karnstein hide in the castle, while their doctor attempts to fight off the angry villagers. He is staked through the heart, and the castle is set on fire.

Lestrange rushes into the burning building to rescue Mircalla. She begs him to leave, but when he refuses she tries to bite him. A flaming rafter falls from the ceiling and impales her through the heart. Lestrange staggers from the inferno into Janet's arms.