Early one evening a shadowy figure climbs up and across a roof top toward the office of Dr. Rufus Maynard. Inside, Maynard tells his partner, Dr. Terry Evans, and Terry's girl friend, nurse Susan Drake, that blood has again been stolen from the laboratory for no apparent reason. When Terry, Susan and the chemist assistant, Fred Mays, leave to run some tests, Maynard is abruptly confronted by the figure from the roof. The man identifies himself as Ormand Murks and reminds Maynard that five years previously, the doctor had placed Murks in Brookdale Mental Institute, under the care of Dr. Garland. Two years later, Garland had brought Murks to Maynard for treatment, but Murks had died on the operating table of no apparent cause. The only malady from which Murks had suffered was an obsessive desire for blood transfusions, which he believed would make him immortal. Murks had requested no autopsy be allowed and that his body be returned to his brother at the Murks estate, Greenwood Knowle. Maynard is astounded by Murks's tale and Murks goes on to tell him that through voodoo rights and devil potions, he has discovered the world between the living and the dead. When Maynard is skeptical, Murks kills him and takes his blood. Later, another figure slips into Maynard's office, going directly to the laboratory storage. Murks comes upon the man, his brother, who is responsible for stealing blood for Murks. When Murks admits he has just killed the doctor for more blood, his brother is angered and decides to call the police until Murks hypnotizes him into submission. Later that same night, two policemen spot a mysterious figure digging near the local cemetery but their approach frightens the man off. They then discover a dead body, which had been strangled and embalmed and which is later identified as Maynard. Back at Maynard's office, police detectives Blair and Hendricks find Terry and Susan and accuse them of murdering Maynard, but with little proof, they depart. Soon afterward, Susan opens the lab refrigerator and the body of Fred falls out. He too has been strangled and embalmed. Blair and Hendricks, returning for further questions, arrest the pair. At headquarters both are grilled for the rest of the night but are released when another strangled and embalmed body is discovered. Returning to the office, Susan finds Murks's identification record in Maynard's waste basket, and she and Terry decide to investigate at the Murks's Greenwood Knowles estate. There, the couple make their way to the family mausoleum and discover that Murks's crypt is open and empty. Murks has seen the couple go into the vault and tries trapping them, but Terry frees them by breaking down the door. Still seeking evidence, Terry and Susan go into the main house, where they discover an undertaking table and the body of Dr. Garland, the head of the Brookdale Institute. Blair and Hendricks, having followed the two to Greenwood, appear and again accuse them of murder. Terry tries to show Blair Garland's body, but it has vanished. After Terry demands that he be allowed to show Blair the suspicious crypt, Susan is left behind at the house, guarded by a policeman. In the mausoleum, Garland's body is found in Murks's crypt while at the house the policeman is knocked unconscious. When Terry and Blair return, they realize that Susan and Blair's car has disappeared. Using the signals transmitted from the car's faulty police radio, Blair and the police track the car to Maynard's office, where they find a hypnotized Susan giving Murks a blood transfusion. Murks makes his escape with the nurse and flees to the rooftop with Terry in pursuit. Just as Murks commands Susan to shoot Terry, Blair arrives and shoots Murks, who topples from the building. Now safe, Susan is reunited with Terry.