The late Dr. Campbell was experimenting with vampire bat blood just before his death. Colleague Paul Beecher (Beal) finds a bottle of pills among Dr. Campbell's effects and takes them home. Dr. Beecher's daughter, Betsy (Lydia Reed), accidentally substitutes the vampire blood pills for her father's migraine tablets. As a result, the kindly Dr. Beecher starts having blackouts from the pills.
During a consultation with patient Marion Wilkins (Ann Staunton), Paul feels unwell and asks her to return the next day. The following morning, he receives a phone call notifying him that Marion has gotten progressively ill. When he goes to visit her, he finds her terrified by his presence, and she dies suddenly. On her neck, Paul finds two puncture wounds.
Worried about his recent blackouts, Paul returns to Campbell's lab where he speaks with his colleague, Will Beaumont (Dabbs Greer). Will tells Paul that Dr. Campbell's research involved regressing animals’ minds to a primitive state, then reversing and advancing the process. The next morning, Henry (James Griffith) is mysteriously found dead with the same puncture wounds and inexplicable disintegration of the tissue on his neck. Later, Paul is called to the hospital to perform an emergency surgery, but is unable to focus and has to leave the operating room.
When Paul realizes he is responsible for the series of local murders—which he has been committing during his blackouts—he arranges for Betsy to stay with an aunt for her own safety. Paul again confronts Will about the pills, but Will assumes Paul is in a delusional state. He agrees to stay with Paul to calm him, and locks the bottle of pills in a drawer. During the night, Will witnesses Paul's transformation into a vampire; Paul then murders Will, disposing of his body in a furnace.
The following day, Sheriff Buck Donnelly (Kenneth Tobey)—suspicious that Will may be engaging in human experiments—goes to question him. At the lab, Buck finds an audio recorder that contains a recording of Will's murder. Meanwhile, Paul's nurse, Carol Butler (Gray), is opening his office when she is confronted by Paul, who urges her to go home. Before leaving, she notices a vial of poison, and realizes Paul may be planning to commit suicide. Before she is able to leave, she witnesses Paul transform into the vampire. Buck arrives shortly after, and witnesses Carol fleeing with Paul in pursuit. She runs into a meadow where she is attacked by Paul, but Buck follows behind and manages to shoot Paul to death. After he dies, his monstrous body reverts to its normal appearance.
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