Boston, 1942. A mother walks into a room and catches her son, Timmy, doing a jigsaw puzzle of a naked female. Very irrigated, she says she is going to burn it. She order her son to get her a plastic bag which to dispose of the remains. Tommy instead takes an axe from a closet and murders his mother. He then saws her dead body into pieces and sits down to finish his puzzle. A woman neighbor shows up at the house and knocks on the front door where there is no answer. After phoning the house and still getting no response, she calls the police. Two policemen force their way into the house and enter Timmy's bedroom where they find blood covering the floor. One of the policemen says "Something's been butchered up here. Let's hope it was an animal". Inside a closet, they find the woman's decapitated head. In another closet in the hallway, they find the blood-splattered Timmy who screams "Mommy!" The policemen console him where he tells the officers that a maniac broke in and murdered his mother.
40 years later. An unseen person enters a fancy apartment and goes to a closet where he recovers a box containing shoes and clothes, mostly covered in dried blood. Along with it is the remains of the nude jigsaw puzzle.
Somewhere on a college campus, a girl is studying in the grass. Nearby, an unknown person is seen working on the shrubbery with a chainsaw. The man dressed in all black clothing approaches and saws off her head with the chainsaw. A little later, Lt. Bracken (Christopher George) and his partner Sgt. Holden (Frank Brana) arrive to investigate the killing. They talk to the college dean (Edmund Purdom) about the chainsaw murder and there were no witnesses and the girl's head is missing. Bracken tells the Dean that they have no suspects and says "We're just out buying clothes without labels and trying them on for size". Willard (Paul Smith) the burly and bearded groundskeeper is outside utilizing a chainsaw similar to the one the killer used.
In the campus library that evening, a girl gives a note to a male student, named Kendall (Ian Sera), expressing the desire to "do it underwater". The unseen killer finds the discarded note nearby and goes to the gym swimming pool where the girl is. As the girl swims, the black-clad killer grabs her by her hair when she swims close to a wall and pulls her out of the water and attacks her with his chainsaw. A little later, Willard arrives and turns on the pool lights where he sees Kendall run from the area and finds a bloody chainsaw and the girl in pieces with her entire torso missing. The police arrive and arrest Willard when he attempts to run. The groundskeeper claims to the cops that he just discovered the body and had nothing to do with the killing. Both Bracken and Holden are stumped by the mutilated dead girl with her entire torso missing. "I wonder what he's doing with all the pieces that are missing" says Bracken.
The next day at the police station, Dr. Jennings (Gerard Tichy) arrives to provide a profile of the murder. Lt. Bracken brings in Mary Riggs (Lynda Day George), a former tennis player-turned undercover-cop. Bracken explains to Holden that Mary is going undercover to pose as a tennis instructor at the college, and Kendall is going to assist her whenever he can.
Sylvia Costa (Isabelle Luque), a Boston Globe reporter, tells Bracken about the rumors she has heard, leading her to believe that a maniac is running loose. Meanwhile, the unseen black-clad killer returns to his apartment and set out to get more pieces of dead women (while continually working on his human jigsaw puzzle). The killer has assembled the girl's severed head to the torso. He now needs a pair of arms and legs.
That evening, the killer (his face still unseen and wearing all black clothing along with black leather gloves with a Fedora hat), stalks the deserted campus and follows a girl who has just finished her aerobics routine. She goes to wait for an elevator and appears to recognize him when he approaches. The girl says "oh, it's you." She tells the killer that she is worried about the two recent killings. The girl and man in black get into the elevator together. Soon, he reveals his chainsaw which has has been hiding behind his back. He starts it up saws both of her arms off (in close-up and graphic ultra-gory fashion). On the floor below, Kendall hears a scream and he and two policemen enter the elevator only to discover that the killer has already fled and the armless woman in the elevator. The police call in for medics, but the woman dies before she can tell them anything about what the killer looked like.
A little later that night, Kendall sees Mary walking around alone at the deserted campus. While the unseen killer is near, Mary has an odd encounter with the kung fu professor, who attacks her and then explains he is not sure what happened after Mary has kicked him in the groin. Afterwords, Mary leaves with Kendall, but Sylvia is still snooping around. The killer follows Sylvia and murders her.
The next day, Mary and Kendall are playing tennis. A fellow tennis player heads into the locker room to shower. The unseen killer appears again and attacks her with his chainsaw. She apparently knows the killer when she screams "You!" The killer chases her into a linen closet where he chainsaws the door down. The killer breaks in and cuts into her torso. At the same time, Mary and Kendall see Willard leaving the locker room and inquire about music being played loudly over the speakers. They eventually find the bloody, dismembered corpse inside the locker room with both of her legs cut off.
Kendall then goes to the police station where he meets Holden and thinks that the killer must be part of the university faculty since the killer seems to know when and where to strike and avoid the police. The look into the Dean's past and they discover that when the Dean was younger, his mother was killed by a person who was never found and that he previously changed his name. They figure out that the Dean must be the killer. They figure out that the Dean must like jigsaw puzzles and has been taking pieces of the dead women to make his own human jigsaw puzzle.
Meanwhile, Mary visits the Dean in his office where he takes her back to his apartment on campus (the killer's apartment). She is clearly in danger of becoming a victim herself. The Dean tells Mary that she has nice feet, so the Dean drugs her drink and plots to kill her to saw off her feet to fully complete his human jigsaw puzzle. Bracken, Holden, and Kendall burst into the Dean's apartment, forcing him to run. They find Mary sitting on the sofa, temporarily paralyzed by the drug.
After the cops leave Kendall there alone, the Dean comes out from behind a curtain (he was only hiding and never fled the apartment) and attacks. Bracken returns and shoots the Dean who falls dead. Afterwords, Holden inadvertently discovers the Dean's human jigsaw puzzle when he leans against a bookshelf and it turns around; the body assembled from the various human parts falls onto Kendall. When Bracken, Holden, Mary and everyone else prepares to leave the scene as the cops take the Dean away in the body bag, Kendall goes for his jacket. Suddenly, the human jigsaw corpse, somehow comes to life and grabs Kendall's crotch and squeezes, vividly castrating him in revenge for her master's death....