Villagers suspect the town simpleton of being a vampire.
Following the sixth mysterious murder in the Central European village of Kleinschloss, villagers fear that a vampire bat has attacked the victims, each of whom has been drained of blood through two puncture wounds leading to the jugular vein. When Martha Mueller, an old apple woman, is killed in the same manner, suspicion falls on Herman Gleib, an eccentric who keeps bats as pets. Dr. Otto von Niemann, whose secretary Ruth loves inspector Karl Breettschneider, tells the dubious Karl of historical evidence concerning bloodsucking bats whose victims became vampires. After another murder, deputized villagers chase Herman into a cave, where he jumps into a pit and dies. Meanwhile, von Niemann's housekeeper Georgiana brings the doctor a crucifix that belonged to Martha, which she found in the servant Emil's room, whereupon von Niemann assures her that he will question Emil. That night, at von Niemann's telepathic command, Emil kills Georgiana, and von Niemann drains her blood in his laboratory. After Karl becomes suspicious, von Niemann give him poisonous sleeping tablets and sends Emil to attack him. When Ruth enters von Niemann's laboratory and witnesses his telepathy, von Niemann, enraged, explains that he has created living tissue which needs food and that a few lives are nothing compared to his achievement. Later, as Ruth, bound and gagged, watches, von Niemann prepares to drain Karl's blood, but when he lifts the cover from the "body," he finds instead of Karl, Emil lying unconscious. Karl, dressed as Emil, pulls a gun, but von Niemann knocks it away. Emil revives and, after hearing von Niemann try to blame him for the murders, kills von Niemann and himself, while the container holding the tissue shatters.
