A young couple are lying on a beach, being watched by an unseen man lurking on the dunes. The woman asks the man to fetch some wood for their fire. While he is gone, the unseen man approaches the woman and suddenly slits her throat with a razor. After gathering some driftwood from the surf, the guy returns to the campfire where the killer attacks him from behind and wraps wire around his neck and pulls, practically decapitating him.
Sometime later, the killer, Frank Zito (Joe Spinell) sits up in his bed screaming. He lives in a small, one-room apartment which is full of bizarre paintings and artwork. There is a framed picture of a woman in a corner of the room decorated with candles and trinkets. There are a few mannequins in the apartment, one of which, wearing the beach girl's clothes and bloody scalp, is lying on the bed beside him. Frank takes off his pajama top and examines some old scars on his chest. He dresses, puts on a heavy winter coat, cloves, takes a gym bag, and leaves into the cold winter night of New York City.
Somewhere in downtown Manhattan, Frank passes by a seedy hotel where two prostitutes are standing outside and one of them asks him if he would like a date. The two of them go inside and Frank checks into a room. In the room, Frank asks the woman if she ever modeled and tells her to show him some poses like the models in the fashion magazines. After she dances a little bit, the hooker and Frank kiss, and she relaxes him after he stops. When they resume kissing, Frank suddenly becomes angry and begins choking her until she is dead. He gets sick and throws up in the nearby bathroom and begins crying and asking "why did you make me do that?" In a very gory and nauseating sequence, Frank pulls out a utility razor and, very slowly and carefully, scalps the woman.
Frank then goes home to his seedy apartment building somewhere in the Brooklyn region of New York. He is carrying a new mannequin to his basement apartment, and tells a passing tenant coming from the building that he is Christmas shopping. In his small one-room apartment, Frank wraps the plastic folding off the new mannequin, puts on the hooker's bloody clothes, and nails the bloody scalp to the mannequin's head, while hearing his own voice telling him that beautiful women whose beauty are crimes punishable by death. Frank pauses and looks at a newspaper headline detailing about the murdered couple at the beach.
Sometime later, Frank dresses up to go out again, this time as a hunter. He disassembles a double-barreled 12-gauge shotgun in a violin case, with some buckshot shells and the utility razor. He puts on his gloves, winter coat, and he tells the mannequins in the room that he will "be right back".
Driving around the nighttime Brooklyn and the Queens area in his beat-up Buick Electra, Frank spots a couple outside a local disco getting into a car and driving off. The man (Tom Savini) and woman park in a lot beside the Brooklyn side of the Verrazano Bridge overlooking the narrows towards Staten Island. The woman from the disco wants to go home, but her pick-up date wants her to stay and view the sight. Frank pulls up near the couple's car, gets out and begins to spy on them as they climb into the back seat to make out with each other. The disco woman stops when she sees Frank by the car window. Upset, she asks the disco man to drive her home. The man climbs in the front seat, starts the car and turns on the headlights and Frank is standing right in front of them. He jumps on the hood of the car, and fires the shotgun through the windshield at the man literally blowing his head apart. The woman ducks in the back seat of the car as Frank approaches, points his shotgun at the woman and fires.
Later, Frank is back in his room with another mannequin made to look like the murdered disco woman as he watches a TV news report about the latest double murder. Frank begins talking to himself and the other mannequins. He mentions stopping the fancy girls and he says they must stop or "they'll take you away from me". Frank then goes to bed and cries himself to sleep.
In Central Park the next day, Frank is walking around when he stops a little girl riding on a bicycle near a playground who accidentally rides into him. He sees a woman (Caroline Munro) in the distance snapping a photo of him beside the little girl. Frank follows the woman, pauses to take a sneak peak at her bag and sees her name and address. The woman is Anna D'Antoni. Frank then walks off.
Later that night, two nurses are leaving the Roosevelt Hospital after their shift and one of them mentions about a killer being on the loose. One of the nurses is picked up by her boyfriend, leaving the other nurse alone outside the hospital. Frank, standing nearby from across the street, begins to follow the nurse. She soon realizes that he is following her and she ducks into a nearby subway station. She gets through the gate, but when she arrives on the platform, the train leaves without her. Seeing Frank approaching, she runs and into a nearby restroom and hides in a stall. Frank walks inside, looks around, and after a minute of tense silence, leaves. The nurse comes out of hiding, looks around the empty and silent bathroom and calms down a bit. Happy that she is okay, the nurse walks to a sink to wash the sweat off her face but when she looks back up, Frank appears standing behind her and stabs her in the back with a bayonet, killing her. Frank washes the blood off the blade. Frank now has another mannequin resembling the nurse as he decorates up the figure while he is play-acting as a hairdresser.
Some days later, Anna is in her darkroom of her apartment developing the photographs she took of Frank when the doorbell rings. Frank is at the door and tells her that he's the man whom she took his photograph in the park. Anna lets him in and they begin to talk about Anna's work as a photographer. He notes that all her models are women, and she tells him that she hopes to sell some of her photographs of the models. Frank tells her that she should keep them to preserve their beauty (it is implied, but never fully explained, that Anna might be a lesbian or bisexual due to her appearance and nature of her photography work). Frank asks Anna out to dinner, and she accepts. During dinner, Frank shows Anna a small photo of his mother in which he tells her that he and his mother lived in Queens until she died several years ago in an automobile accident. Anna reveals that she is from Italy but has done photography work mostly in England. Frank asks Anna if she would like to go out with him again and she invites him to an art gallery show for her photographs on Thursday.
Another few days later, Frank arrives at a studio where Anna is taking photos of three models at a photography session. After taking a break, Frank gives Anna a teddy bear as a gift and she introduces him to one of the models named Rita. Anna and Rita go back to shooting, and Frank eyes the two women casually talking and holding hands together (again implying, but not fully revealing, that Anna and Rita's friendship is more then that). Frank impulsively steals a gold necklace that Rita set aside and leaves.
Later that evening, Rita arrives home at her West End Avenue apartment, looks through her mail, and begins to run bathtub water to take a bath. Rita answers a buzzing at the front door and it is Frank who returns her necklace (claiming that he found it at the studio). With Rita distracted for a second, Frank slyly pushes in the door's lock, and leaves. Rita goes into the bathroom to take her bath. A little later, Rita is making tea for herself when she is grabbed by Frank who jumps out of a closet. When Rita wakes up, she finds herself gagged and tied to her bed. Frank then rambles at Rita saying that her hair is different, but he knew he could find her. He begins talking to her about abuse and leaving him, implying that he is talking to his mother. He tells Rita that he is going to keep her so she will never go away again. Frank then pulls out a switchblade knife and after a minute, stabs Rita in the chest, killing her. Frank then begins to scalp her as well and adds her hair and bathrobe as another trophy to his growing number of mannequins he keeps in his apartment. In his apartment, Frank sits in front of the woman's picture, which is his late mother, and talks about hiding in the closet when his mother said so.
Some days later, Frank calls Anna and invites her to a show and picks her up. Anna thanks Frank for sending flowers and attending Rita's funeral. Frank drives Anna to a cemetery and says that he wants to put a reef of flowers on his mother's grave which she died on Christmas Day. At the cemetery, Frank lays the reef beside the headstone of his mother, Carmen Zito, and begins to cry. He says that Rita knew about him and he suddenly grabs Anna by the neck. Anna hits him and runs, and Frank chases her through the cemetery. Anna wounds Frank by jumping out from behind a gravestone and hits him in his left arm with a shovel and flees out of the cemetery. Frank screams out in pain and calls out "Mother!" Frank then begins to hear voices in his head and the voice of a young Frank. He is pleading with his mother not to lock him up in the closet. It is fully revealed here that Frank was abused and neglected by his own mother, leading him to go on this killing spree. Frank returns to his mother's grave and kneels at the headstone crying again until suddenly the rotting corpse of his mother bursts out of the ground and grabs him (shades of the movie 'Carrie'). But it was all just a hallucination, and Frank sits back alone at the grave and cries some more.
Frank goes home to his apartment and lies on his bed with his wounded arm crying over his mother. As he looks around the room, the mannequins suddenly turn into the bloodied murdered women they represent and they all approach Frank with weapons they pick up off his table. In the most goriest and horrific scene, the mannequins/zombies attack him, one stabbing him in the stomach with the bayonet, one chops off his right arm with a machete, and two of them literally rip off his head from his body in a ghastly flow of blood and gore, splattering all over everything.
The next morning, a police car arrives outside the apartment building where Frank lives. Two plainclothes police officers, having obviously been called upon by Anna, get out, pull out guns, and burst into Frank's apartment. They find Frank lifeless on his bed (with only a knife in his bloody stomach), with all the mannequins standing around the room. Apparently the whole supernatural event was nothing but Frank's horrible nightmare which resulted in him committing suicide. The two cops leave the room, and Frank continues to lie motionless on his bed... then his eyes open...