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The murder of the wealthy Sir Karell Borotyn in the study of his Czechoslovakian castle sparks renewed interest in a local legend that claims that the place is haunted by corpses that rise up at night to suck the blood of mortals. Borotyn's body, found slumped over his desk, drained of its blood and marked with a distinct bite mark on the neck, is examined by Dr. Doskil, who quickly asserts that Sir Karell was killed by a vampire's bite. After dismissing Doskil's conclusion as mere superstition, Inspector Neumann begins his investigation of the crime by questioning suspects Fedor Vincente, who stands to gain a large sum of money from Sir Karell's death as the fiancé of his only child Irena, and Baron Otto, Irena's new quardian and soon-to-be executor of her estate. The investigation continues in a doctor's office, where the coroner questions a local innkeeper, who believes in the legend and says that the last time he saw the infamous vampires, Count Mora and his daughter Luna, they appeared as bats in the night. When the learned Professor Zelin, an expert in the occult, learns of the circumstances surrounding Sir Karell's death, he, too, expresses his belief that Karell was killed by a vampire. Next to encounter the mysterious forces of the castle legend is Fedor, who enters Irena's house in a state of obvious distress, unable to remember anything other than running by the old castle to catch a train then falling down. Dr. Doskil attends to the fatigued Fedor, on whom he finds a bite mark that is identical to the one found on Sir Karell. Later that night, a horse-led wagon rushes through the night carrying Maria, a new servant at the Borotyn estate, and as she and her driver pass by the old castle, they are halted by the frightening presence of Luna walking in the mist. Maria returns to Irena's in a hysterical state, but Baron Otto insists that she merely had an encounter with real estate agents showing the property, and not the vampiress she claims to have seen. When Irena is found inexplicably sitting in the garden in the middle of the night, it is suspected that she, too, has been attacked by Luna, and Professor Zelin orders that bat-thorn, a weed that wards off supernatural spirits, be spread throughout the household and that all the windows and doors remain shut. Though it has been a year since Karell's death, his name is found on a recently signed lease for the castle, prompting an inspection of Karell's coffin, which is discovered to be empty. While the help spreads more bat-thorn around the house, a bat flies in and is followed by the apparition of Count Mora. After learning that the only way to rid the house of vampires is to find their graves during the day, slice their heads off and place bat-thorn in their necks, Inspector Neumann, Baron Otto and Professor Zelin form an expedition into the depths of the castle foundation to perform the deed. While sneaking up to one of the castle windows, Inspector Neumann and Baron Otto witness the strange sight of Sir Karell in the salon with Count Mora and Luna. When Irena arrives at the castle and complains that she can no longer go through with Professor Zelin's plan to force Baron Otto's confession of her father's murder by frightening him with the sight of his ghost, it becomes apparent that the supposed supernatural occurrences of the past year were all part of a hoax designed by Zelin to crack Sir Karell's murder case. Having failed in his effort to scare the baron with Karell's apparition, he then hypnotizes the man and, with the help of an actor posing as Sir Karell, initiates a re-enactment of the events leading up to the murder. The conversation that the two men had prior to the murder reveals that the baron forbade Karell's consent to the marriage of his daughter to Fedor. Having secured the motive of the crime, Zelin and Neumann observe the baron as he prepares to murder the actor posing as Karell, but stop him once they discover his blood-letting technique. Irena apologizes to Fedor for not letting him in on the scheme, and Count Mora and Luna remove their costumes and plan their next stint as part of their haunted theater act.