On the outskirts of London, in 1918, Andreas Obry, a werewolf, awakens his master, Hungarian vampire Armand Tesla, to report that the vampire's latest victim is being kept alive at a nearby sanitarium run by Lady Jane Ainsley. At the sanitarium, Lady Jane and her friend, Professor Walter Saunders, examine a slide of blood drawn from their nearly bloodless patient. After the woman dies hysterically screaming, Saunders reads an article by Hungarian Tesla, a respected scientist, about the dead returning from the grave. As Saunders reads the treatise, Tesla steals into the professor's granddaughter Nicki's room. The next morning, Saunders discovers two puncture marks on the girl's throat and realizes that she has become the vampire's latest victim. Knowing that Nicki's salvation lies in the destruction of the vampire, Saunders and Lady Jane set out to find the creature's grave. At the burial ground of the old priory, they trace footsteps to a coffin. After opening the lid, they drive a stake through the creature's heart just as Andreas enters the mausoleum and is transformed back into a human by the vampire's death. Twenty-three years later, Andreas serves Lady Jane as her faithful lab technician, and Nicki is engaged to Lady Jane's son John. Saunders has died in a plane crash. When Lady Janes tells Sir Frederick Fleet, the head of Scotland Yard, the story of Tesla and shows him Tesla's manuscript, the inspector insists upon opening the vampire's grave to investigate. Before they can proceed to the cemetery, however, a bomb that was dropped during a German air raid unearths the vampire's grave. While cleaning up the rubble, two municipal workers discover Tesla's body, pull the stake from his heart and then bury him. When Sir Frederick discovers that the graveyard has been hit by a bomb, he decides to drop the inquiry. That night, Tesla rises from the dead and summons Andreas to his grave. There, he reclaims Andreas as his servant and transforms him into a werewolf. Plotting revenge on Lady Jane through her son, Tesla sends Andreas to eliminate Dr. Hugo Bruckner, a scientist who has been smuggled out of Germany by the London underground. After Andreas returns with Bruckner's papers and clothes, Tesla assumes the dead man's identity. At an engagement party for Nicki and John, Tesla appears at the sanitarium and after he introduces himself as Bruckner, Lady Jane offers him access to her lab. That night, Tesla re-establishes his power over Nicki. The next morning, Lady Jane discovers Nicki unconscious with two puncture marks on her neck. Hurrying to the cemetery, she questions the workers, who tell her about the corpse they found with a spike through its heart. When she discovers that the body is now missing, Lady Jane is certain that the vampire lives. Although Sir Frederick listens to her story with skepticism, he dispatches two of his men to follow Andreas. When the men stop Andreas for questioning, he turns into a werewolf and flees. That night, Tesla orders Nicki to attack John. When Lady Jane discovers her son's unconscious body with puncture marks on his throat, she and Sir Frederick go to the lab to question Andreas. After Andreas flees when they discover his fur-covered hands, they decide to question Bruckner and proceed to his room at the men's club. The room is deserted, and when Lady Jane notices that the mirror has been turned toward the wall and finds Tesla's ring there, she deduces that Tesla is posing as Bruckner. As Lady Jane plays the organ at the sanitarium that night, Tesla visits her and taunts her about his power over Nicki. In response, Lady Jane calmly challenges Tesla with the power of faith and goodness and flashes an image of a crucifix at him, driving him away. Soon after, however, Sir Frederick arrives and he and Lady Jane watch helplessly as Nicki descends the stairs, summoned by Tesla's voice. Knowing that Nicki will lead them to Tesla's grave, Lady Jane insists on following her. At the cemetery, Sir Frederick sees Andreas hiding in the bushes and shoots him. At that moment, German bombers attack and Nicki is struck down by falling debris. Andreas picks her up and carries her to Tesla, and then begs for help with his wound. When Tesla refuses help and denounces him, Andreas finds a crucifix in the rubble and, now imbued with the power of goodness, becomes human again. After Andreas weakens Tesla by showing him the crucifix, a bomb falls, rendering the vampire unconscious. Pulling the vampire's body into the sunlight, Andreas drives a stake through his heart, thus freeing Nicki from his power. When Sir Frederick and Lady Jane locate Nicki, they find Andreas' body lying next to Tesla's skeleton.