In San Francisco, a late-night discovery sets off a chain of unsettling events. The elderly Dr. Campbell had been toying with vampire bat blood as part of his experiments, and just before his death he leaves behind a clue in the form of a bottle of pills. Colleague Paul Beecher brings those pills home, hoping they hold the key to alleviating his patients’ pain, but fate takes a dark turn when his daughter Betsy accidentally swaps his father’s migraine tablets for the vampire blood concoction. The result is a troubling shift: Paul begins experiencing powerful blackouts that blur the line between doctor and danger.
During a routine consultation with Marion Wilkins, a patient battling congenital heart problems, Paul starts to feel the strangeness creeping in again. He asks Marion to reschedule, but the next day a chilling report arrives: Marion has fallen gravely ill, and when Paul responds to the hospital, he finds her terrified by his presence and dead with two puncture wounds on her neck. The mystery deepens as Paul returns to Campbell’s lab and encounters the university’s psychiatric head, Dr. Will Beaumont, who oversees the study, accompanied by the detached researcher Henry Winston. Beaumont explains Campbell’s broader aim: to regress minds to a primitive state and then reverse the process, a theory that promises to unlock newer levels of intellect. Yet the pills remain an enigma, and Paul, hungry for answers, finds himself increasingly unmoored.
The plot thickens as Henry is found dead in the same grisly fashion, with the same neck wounds and a disturbing tissue disintegration. Paul is summoned to perform surgery but finds himself distracted and unable to concentrate, forcing him to abandon the operation mid-procedure. As the moon grows full in the background of a creeping dread, Paul confronts the terrifying possibility that he himself is responsible for the killings that have shaken the community—acts he cannot recall committing during his blackouts. In a bid to protect Betsy, he sends her to stay with relatives, while he confronts Beaumont about the pills. Beaumont, though, suspects a delusion and locks the pill bottle away, attempting to steady Paul and prevent further harm.
That night, the transformation finally manifests in a brutal, undeniable form: the pills turn Paul into a vampire, propelling him to murder, and he wastes no time in disposing of Will’s corpse in a furnace. The investigation widens as Sheriff Buck Donnelly presses questions about Will’s experiments. He discovers an audio recording that confirms Will’s murder, weighing heavily on the case. Meanwhile, Paul’s nurse, Carol Butler, becomes unwittingly entangled in the nightmare as she checks Paul’s office. She notices a vial of poison and understands he might be contemplating suicide, but her alarm escalates when she witnesses Paul’s sudden change into a vampire. Buck arrives just in time to see Carol flee, pursued by Paul through the night.
The chase culminates in a tense wooded chase, with Carol escaping and Buck standing between the hunter and his quarry. As the danger closes in, Paul’s pursuit turns violent, and he fights Buck before a final intervention arrives: Officer Ryan shoots Paul, sending him tumbling into a shallow culvert. In a haunting final twist, the monstrous form fades, and Paul’s body reverts to its normal appearance as he dies, leaving behind a stark reminder of the vampire’s curse and the price of unrestrained science.
