Dr. John Beck and his wife, Cathy, travel to a mountainous area where John plans to do research. On a guided tour inside a cave, John’s note-taking is interrupted when Cathy pulls him aside to suggest they make love. In search of a more secluded spot, Cathy slips and falls into a swarm of mites. John rushes to her aide as a bat swoops down and attacks them both. Though Cathy is unscathed after the attack, John’s head is bleeding. Soon after, John and Cathy go skiing, and on a gondola ride to the top of a mountain, John experiences an hallucination in which he sees bats chasing after Cathy. When John regains consciousness, he cannot remember the episode, but Cathy and the other skiers on the gondola are shaken by his strange behavior. After a day of skiing, Cathy and John relax in a hot tub, but John gags on his drink and cuts his hand while gripping the glass. Worried that he might be rabid from the bat attack, John goes to see Dr. Kipling the following day to procure the Pasteur treatment, a vaccine for rabies. Moments after Dr. Kipling administers the vaccine, however, John convulses from an allergic reaction to the medicine. Later, at the hospital, he wakes in the middle of the night and sees that his hand has become webbed and grown claws. John lurches out of his room and attacks the night nurse. The next day, another nurse tells John that the night nurse had an accident, and he remains mum about the attack. Cathy delivers flowers to John and suggests moving him to a nearby hotel so they can stay together, and John reluctantly agrees. In town, John suffers another episode as he buys a gift for Cathy. Sergeant Ward, the town sheriff, walks into the shop in search of John. As John covers his hand, worried that it might have become webbed again, Ward brandishes John’s hospital bracelet that was found near the night nurse’s body and suggests that John is a suspect in the attack. Sleeping in a hotel with Cathy that night, John wakes from another nightmare and heads to the street where he steals a mannequin from a shop, beats it, and steals its sweater. At a nearby trailer park, John stalks a young couple in a truck, and as the young man leaves, John chases the girl. He later wakes from a bad dream, unable to determine whether the attack was real. Shortly after, Sergeant Ward arrives at John and Cathy’s hotel room and informs John that a young girl was attacked nearby. Ward dangles a bandage for John to see, saying that the girl ripped it off the hand of her attacker and implying that the bandage is the same one he saw on John’s hand earlier. That day, John returns to Dr. Kipling’s office and receives a prescription for sleeping pills, but John worries that his nightmares have all been real. John checks into the hospital again for monitoring, and another episode starts when Cathy visits. John sends his wife away and sneaks outside to steal an ambulance. As John speeds through town, Ward and other police pursue him. John veers off a road, crashes, and escapes on foot. He takes cover in an abandoned shack where he meets a drunk hobo; the hobo offers John a swig, and John gags on the alcohol. Hearing bats, John convulses, and, after transforming into a hybrid bat-human, kills the hobo. The next day, John traverses the desert and, from inside a cave, the sound of bats beckons to him. Meanwhile, Ward continues his search for John and visits Cathy at her hotel room for an update. Cathy defends her husband, insisting he did not commit any of the attacks, and Ward responds by kissing her aggressively, but Cathy pushes the sheriff away. Back in town, John breaks into the hospital to steal more rabies vaccine. Though he subdues a nurse in the process, John becomes distracted by a bag of blood. As he goes to drink the blood, the nurse runs away, finding Ward in the hallway and sending him after John. However, John escapes before Ward can catch him. Later, the police find an audiocassette John recorded while alone in a cave. On the recording, John explains that he is transforming into a bat and says goodbye to Cathy. He also states that he has a sharper awareness now and that his metamorphosis has freed him from negative human thinking. Later, Ward plays the audiocassette for Cathy, but she orders him to turn it off, arguing that John has been negatively affected by the rabies vaccine and is therefore incapable of reason. That night, John visits Cathy in her hotel room, speaking to her from behind a door. He tells her nothing can be the same and asks her to return home. When Cathy suggests he come home with her, John says the caves are his new home. He emerges from behind the door and Cathy turns on the light, trying to convince her husband that his metamorphosis is only an allergic reaction. She kisses John, and, later, as they make love, John’s face becomes bat-like. Cathy opens her eyes and screams, causing the hotel manager to barge into the room. Upon his arrival, however, Cathy is alone, appearing to have just awakened from a nightmare. The next day, Ward finds John inside a cave and beats him into submission, but John morphs into a bat creature and fights back. Later, Ward suffers a head injury from the fight with John, and his face becomes disfigured as the injury swells. Cathy joins Ward as he drives back to the cave, but a swarm of bats attacks the car. Unperturbed, Cathy bids Ward goodbye as she gets out of the car, allowing the rabid bats to fly inside and attack the sheriff. Sometime later, Cathy wanders into a cave as if she is being beckoned.