Dr. John Beck and Cathy Beck are newlyweds on a delayed honeymoon in southeastern California, and their trip quickly tilts from romantic to eerie. While they pause to tour Mitchell Caverns, a fruit bat has landed on their picnic blanket, and Dr. Beck, who specializes in bats, manages to frighten it away as Cathy watches with growing unease. On a guided tour inside the cave, he hears the strange noises again and quietly warns that “the bats … one of them is getting closer.” Soon after, he is bitten on the forehead by the approaching creature, a bite that marks the beginning of a terrifying transformation.
The symptoms begin as mild seizures, and Cathy grows more worried as the odd tremors escalate. Later, while sharing drinks in a motel jacuzzi, Dr. Beck endures a violent seizure that shatters a drinking glass from his grip. A local physician, Dr. Kipling, notices the wound and worries about rabies, prescribing the Pasteur treatment as a precaution. Cathy rebukes him for doubting the idea at first, and Beck reluctantly agrees to start the injections.
At the hospital, the first of twenty inoculations triggers another brutal seizure. Nurses whisper that the seizures are not linked to rabies since the incubation period typically spans months. That night, in his hospital bed, Beck undergoes a full transformation into a human-bat creature. The next morning, Cathy brings flowers, but Beck is already unsettled and suggests a psychiatrist might be needed, a suggestion he ultimately rejects after a heated exchange.
A shopping trip becomes a turning point. While looking for a gift for Cathy, Beck experiences another seizure, and a nearby sheriff, Sgt. Ward, witnesses it from outside the store. Ward explains he’s investigating an attack on a nurse tied to his hospital room, flashing his patient ID bracelet as evidence. That night, Beck steals clothing from a mannequin and kills a woman in a trailer park. Cathy insists it was only a dream when Beck wakes screaming in their motel room, and Ward appears again to question him about the bandage found near the murder scene.
The medical side presses on, but the danger deepens. Dr. Kipling tries to downplay the recurring nightmares as mere anxiety, while Beck endures another seizure in his hospital bed and escapes through a window, hijacking an ambulance. Ward pursues but crashes in a ditch; Beck hides in a barn, tends to a homeless man’s injured hand, and samples the man’s booze before another seizure drives him to kill again. Beck returns to Mitchell Caverns and, in a full nocturnal frenzy, kills a tourist. Realizing the need for blood, he returns to the hospital to access the stock in the facility’s refrigerated area and escapes just before Ward arrives.
Beck records a confession, then the relationship with Cathy intensifies. He composes a confession detailing his crimes, the ongoing transformation, and some of his philosophical thoughts. Cathy, hearing the playback, refuses to believe him and blames the Pasteur treatment for his erratic behavior. Later, she finds Beck in the motel again, and she professes her love. In a chilling turn, she is bitten by Beck while they lie together.
The pursuit climaxes at Mitchell Caverns. Ward tracks Beck to the cavern, and in the ensuing struggle, the sheriff is disarmed but manages to flee back to his department. While Ward receives medical attention, Cathy begins to hear the same unsettling noises Beck did. The pair drive back to the motel, only to be ambushed by a swarm of bats that attacks Ward’s car. Cathy watches from the passenger seat as the vehiclecareens into a ditch, exits the car, and coldly states, “Goodbye, sheriff.” The mass of bats swarms Ward and kills him, and Cathy finally embraces her fate as a human-bat creature, joining Beck in the cave.
