Sir Karell Borotyn [Holmes Herbert
] has been murdered just two weeks before the wedding of daughter Irena [Elizabeth Allan
] to fiance Fedor Vincenti [Henry Wadsworth
]. Sir Karell's body was found with two marks on his neck and drained of blood. The superstitious villagers suspect vampirism and finger the town vampires Count Mora [Bela Lugosi
] and his daughter Luna [Carroll Borland
], but Police Inspector Neumann [Lionel Atwill
] can't buy that. After months of investigation, however, Sir Karell's death is declared 'cause unknown', Irena goes to live in the house of guardian Baron Otto von Zinden [Jean Hersholt
], and the wedding plans continue as before.
One morning, Fedor stumbles in with two marks on his neck, complaining that he fell and hit his head outside of the now-deserted Borotyn castle where Mora and his daughter are supposedly living. Irena is attacked one evening by Mora's daughter, and Neumann calls in vampire expert Professor Zellen [Lionel Barrymore
]. When Sir Karell's signature is found on a lease signed after his death and when Irena claims to have heard her father's voice, Neumann and Otto dig up Sir Karell's coffin and find it empty. Shortly thereafter, the maid and butler see Mora in the house and the Professor sees Mora and Sir Karell walking together in the garden, so Neumann and Otto go snooping at the castle. There, they find Mora, his daughter and Sir Karell. That night, Mora's daughter again attacks Irena. Determined to find and destroy the vampires, Neumann, Otto, and the Professor go to the castle, but Mora's daughter leads Irena there, too. Fedor follows and almost destroys the plan.
The plan: The Professor (actually Chief Inspector of Prague Police) hypnotizes Otto into re-enacting Sir Karell's murder. It turns out that Otto drugged Sir Karell and murdered him because he wanted Irena for himself. Mora, his daughter Luna, and "Sir Karell" turn out to be actors from Luna's Bat Woman Theatre, hired by the police to ferret out Otto. [Original synopsis by bj_kuehl.]