In San Francisco, the shadowy path of horror begins after the celebrated but reclusive Dr. Campbell dies while pursuing experiments with vampire bat blood. Colleague Dr. Paul Beecher discovers a bottle of pills among Campbell’s effects and takes them home, unknowingly stepping into a nightmare of blood and mind. Campbell’s daughter, Betsy Beecher, misplaces or misuses the pills and substitutes vampire-blood capsules for her father’s migraine medicine. Almost at once, Beecher the elder begins to experience alarming blackouts, a creeping loss of control that unsettles his patients and colleagues alike.
During a routine consultation with a patient named Marion Wilkins—who battles a serious congenital heart condition—Beecher himself feels unwell and urges Marion to return the next day. The following morning brings a chilling development: Marion has fallen gravely ill, and when Beecher arrives at her room, she is visibly terrified by his presence, collapsing soon after. A distinctive pair of puncture wounds marks her neck, a sign that something monstrous may be at work.
Desperate for answers, Beecher returns to Campbell’s laboratory, where he runs into the university’s psychiatric chief, Will Beaumont, who oversees Campbell’s controversial research. Beaumont is accompanied by the aloof researcher Henry Winston, who helps interpret Campbell’s progress. Beaumont explains that Campbell’s work sought to regress animal minds to a primitive state and then reverse the process to sharpen intellect—a notion Beecher can barely grasp but cannot ignore. As evening falls, Beaumont retires to a nearby hotel, leaving Beecher alone with his questions about the pills. The next morning, Henry Winston is found dead in much the same way Marion was, his neck bearing the same puncture wounds and his tissues starting to disintegrate, a ghastly deja vu.
The pattern of horror intensifies when Beecher is called to an emergency operation he cannot complete due to a failure to maintain focus in the sterile corridor. Plagued by the blackouts, he confesses to himself that he may be responsible for a string of local murders. To protect Betsy, he sends her to stay with an aunt, stepping back from the danger that lurks in his own hands. He confronts Will Beaumont again, but the psychiatrist dismisses Beecher as delusional. Will agrees to stay by his side, only to lock the pill bottle away in a drawer, perhaps hoping to calm Beecher’s fear.
That night, the transformation Beecher has dreaded becomes a brutal reality: he becomes a vampire and murders Will, disposing of the body in a furnace. The next day, Sheriff Buck Donnelly—now suspicious that Will may have been conducting human experiments—visits the lab and uncovers an audio recorder containing a recording of Will’s murder. At Beecher’s office, his nurse, Carol Butler ( Beecher’s secretary and companion), notices a vial of poison and begins to suspect that Beecher might be plotting suicide. Before she can leave, she witnesses Beecher’s full transformation into a vampire. Buck arrives and sees Carol fleeing with Beecher in pursuit. Carol escapes into a nearby wood, where Beecher closes in and attacks her; Buck rushes after, but Beecher fights back and overpowers him as well.
The chase culminates when Officer Ryan, Sgt. George Ryan, arrives on the scene and shoots Beecher just as the monstrous creature collapses. The body tumbles into a shallow culvert, and as Beecher dies, the monstrous form quickly reverts to a normal human appearance, leaving a grim echo of what the city just endured. The town’s nightmare ends with the unsettling realization that the line between science and something far darker had been crossed—and that a single act of carelessness could unleash consequences beyond anyone’s control.
