Charles Brady and his mother Mary are Sleepwalkers — nomadic, shapeshifting energy vampires who feed off the lifeforce of virgin women. Though they normally maintain a human form, they can transform (partially or fully) into human-sized bipedal werecats (presumably their natural form) at will. They are considerably more resilient than humans and have powers of both telekinesis and illusion. Their one weakness is cats, who are not only able to see through their illusions but whose claws are capable of inflicting severe to fatal wounds upon them. They also maintain an incestuous relationship.
Charles and Mary have taken up residence in a small Indiana town, having recently fled Bodega Bay, California (where they used the aliases Martha and Carl Brodie) after draining and killing a girl there. Charles attends the local high school, and there he meets Tanya Robertson in his creative writing class. Tanya does not suspect the real reason why Charles wants her so much — to take her lifeforce for his mother, who is starving. At first, it seems that Charles has fallen in love with Tanya (to the dismay of his jealous mother). On their first date, however, a picnic at the nearby cemetery, Charles attempts to drain the lifeforce from Tanya while kissing her. Tanya tries various things to ward off Charles' somewhat unusual advances, including bashing his head with her camera, scratching his face, and ultimately plunging a corkscrew into his left eye.
As it happens, Deputy Sheriff Andy Simpson who had earlier tried to pull Charles over for speeding and passing a stopped school bus (and seemingly trying to run down a young school girl), drives by the cemetery and notices Charles’ (now undisguised) car. When Tanya runs to him for help, Charles tracks Andy down and kills him. When Charles then turns to resume his lifeforce-depleting makeout session with Tanya, the deputy’s cat, Clovis, rises to the occasion and nearly kills Charles by scratching him in the face and chest. Mortally wounded by Clovis, Charles staggers back home to Mary who is able to make both of them "dim", i.e., turn invisible, and thus keep Charles from being arrested when the police storm their house.
Mary then seeks vengeance on the Robertson family, killing the sheriff and his entire department and fatally wounding Tanya’s parents. She kidnaps Tanya and takes her back to her house, where Charles by this time is all but dead. Mary resurrects him, and Charles tries to drain Tanya’s lifeforce. However, Tanya plunges her fingers into Charles’ eyes, killing him. Tanya manages to escape to safety with the help of the sheriff who is later impaled on the picket fences surrounding the house by Mary. Several of the cats that had been gathering around their house throughout the movie, led by Clovis, jump on Mary and claw at her until she bursts into flame, killing her. The movie ends with Mary lying ablaze on her driveway and Tanya hugging Clovis, her savior.