At the Jack-n-Jill Daycare Center, teachers aide Anne Barron (Meb Boden) is saying goodnight to the young child Lisa who gets picked up by her mother. Anne sits alone on the merry-go-round when a person wearing all black leather with a black motorcycle helmet, rides up on a motorcycle to the daycare center. After looking around and seeing that Anne is alone, the black-clad person accosts Anne by spinning the merry-go-around and holds out a kukri at Anne as she passes, until finally the blade is lifted and Anne screams.
The next morning, Boston Police Lieutenant Judd Austin (Leonard Mann) is with his girlfriend Stevie (Margo Skinner) on his day off, but his partner Taj (Joseph R. Sicari) calls him and Judd leaves to go to his "lousy job". He arrives at the scene of the crime and is told by the coroner that the murder of Anne Barron is similar to an earlier one the previous month where a young woman was decapitated. Judd is shown Anne's severed head, resting in a nearby bucket of water. The previous victim's head was found in a lake. The director of the daycare center, Miss Armand, tells the cops that Anne attended evening classes at Wendell College, a local girls school. Judd sets off to try to find a meaning in the decapitated heads in the water.
Judd goes to the school and stands in the back of Professor Millett's (Drew Snyder) anthropology class as the teacher is finishing a lecture. When the class is dismissed, Judd approaches Millett to ask him questions about Anne, and Millett says that his student Kim Morrison (Elizabeth Barnitz) was Anne's friend. Kim is clearly distraught when she learns about Anne's murder and is consoled by Millett. Eleanor (Rachel Ward), a British exchange student at the college, walks in and tells Millett that she has finished his notes. Kim tells Judd that Anne had a boyfriend but she would not tell her who it was.
That evening, Eleanor goes to the Lamplight Restaurant near the college for some coffee where the weird busboy Gus (Nicolas Cairis) stares at her. Carol (Karen McDonald) a middle-aged waitress, asks Eleanor if she is in Millett's class and if the teacher "fools around". Eleanor evades talking about Professor Millett and she instead pays for her coffee and leaves. Eleanor is walking home alone when she is frightened by the shadowing Gus who follows her. She runs and gets inside a row house and locks the doors. Eleanor begins to take a shower when someone begins rigging the front doorbell, but Eleanor doesn't hear the bell. A few minutes later, a man walks in through the back door and walks upstairs to the bathroom where Eleanor is scared by Millett. The two of them kiss and then enjoy themselves in the shower as the professor paints his student's nude body. It is clear that Eleanor and Millett are lovers and live together in the professor's house.
The next day, Kim is in scuba gar at the local aquarium feeding fish and turtles as people watch through the aquarium glass. The girl gets out of the water and goes to the locker room to take off her scuba diving suit when the black helmeted killer jumps out of a locker and slashes her several times with his kukri until he finally beheads her and drops her severed head into the tank so the people can see.
After leaving the latest crime scene, Judd goes to see Millett at his home where Eleanor is there, saying that she is the professor's live-in "research assistant". Judd goes to the university where he finds Millett in his office to question him about the latest beheading murder, whom was another one of his students he questioned the previous day. Millett says that the only animal that kills for pleasure is man. Judd tells Millett that Kim used to be his student and asks Millett if he had an affair with Kim or any other student. The teacher denies that he is having affairs with any of his female students as well as being responsible for the murders. With no evidence to arrest him, Judd leaves but he tells Millett: "People don't go out and kill just for the hell of it. There's always a reason."
After Judd leaves, Millett meets with a tearful Eleanor where she tells him about Judd visiting her at the house earlier and that he thinks that Millett is responsible for the murders. Millett again denies that he's responsible for the killings and Eleanor seems to belive him. Eleanor tells Millett that she loves him and will do anything for him, but there is always another person. Eleanor walks out and Millett catches up to her at the Lamplight Restaurant and apologizes, and he again denies having affairs with other women or being the one responsible for the killings. Eleanor tells him that she believes that he is innocent of the murders, but not innocent of cheating on her. She tells Millett that she is about a month pregnant. They both seem happy about it, but Eleanor seems a little irked when Millett flirts with the waitress Carol a little bit.
Millett goes to see Helen Griffin (Annette Miller) the dean of the school, and he hears her talking to Kathy (Holly Hardman) another student about allegations of having an affair with the professor. Helen discusses Millett losing his job, and she offers to let Kathy stay at her place.
That evening, Carol is closing up the restaurant for the night after the manager Gary (Bill McCann) leaves. Suddenly the lights in the restaurant are turned off and Carol naturally asks: "Who's there?" Just then the same black-clad killer appears and the kukri is brought out. Carol manages to get out the back door, but the killer catches up, swings the kukri, and carries her severed head back to the restaurant. The next morning, Gary opens up the restaurant for a couple of construction workers and they see the mess of overturned tables and chairs, but Carol is nowhere to be seen. Gary serves them two bowls of beef stew for breakfast and one of them notices a strand of hair in his bowl. Gary examines a pot of stew that had been left on the stove and empties it out. Then, he notices a nearby sink full and upon draining it, discovers Carol's severed head. Gary calls the police where they find Carol's decapitated body in the back alley in a trash bin. Judd and Taj show up where Gary tells them that he suspects it was the mentally challenged busboy Gus who did not show up for work and gives them Gus' home address.
Judd and Taj go to Gus' seedy apartment and question him where Gus tells them that he didn't go to work because he was "too sick". Taj finds a bra in Gus' drawer, but Judd decides to leave, thinking that Gus is only a peeping tom.
Judd goes back to Millett's house and when no one answers the door, he picks the lock and lets himself inside. Looking around for clues, Judd goes into Millett's office where he finds photographs of Millett and Eleanor posing with tribal people. Eleanor comes in and asks the cop if he has a search warrant. He tells her 'no' and he asks Eleanor about headhunting (implied by the photographs). Eleanor tells Judd that during her and Millett's travels to New Guinea, heads were cut off of criminals because the people believed they could process life force from the heads of enemies; the heads were put in water to cleanse the evil spirits.
After Judd leaves, Eleanor goes over to the school to find Millette to apparently inform him about Judd's persistance. Unable to find Millette in his classroom, Eleanor walks into Helen's office where she interrupts a discussion between Millett and Helen about the professor having an affair with one of his students. Clearly angry and jealous, Eleanor storms out, and Millett is angry with Helen, suggesting that her interest in the students is something other then "welfare".
That evening, Judd is staking out Millett's house where he follows Millett as he leaves his house on his motorbike, but loses him at a red traffic light. Just then Taj radios to Judd that he has been staking out Gus' place and that he has followed the peeping tom whom is right outside Helen's house. Gus is peeping inside the windows while Taj is a short distance away. Inside the house, Helen is laying in her bed with Kathy clearly trying to seduce her. Just then a telephone rings, and Helen goes downstairs to answer it where she is grabbed by the killer who is already inside the house. A few minutes later, Kathy wakes up and enters the bathroom where she finds Helen's severed head in the toilet. Kathy is then beheaded by the same killer who puts her severed head in the filled bathtub.
Just then, Judd arrives and bursts down the front door where he finds Helen's headless body in the living room. In going upstairs to the bedroom, the killer jumps out at Judd, knocking him down the stairs. The killer then runs out the front door, gets on a motorbike, and speeds away. At the same time, Taj arrests Gus outside for trespassing. Judd stumbles outside and tells Taj that he strongly suspects who the killer is and they need to follow him.
The black leather-clad killer returns to a place at dawn and enters through a back door and takes off the helmet. It is Eleanor. Millett walks in and she shows Millett the bloody kukri saying: "I did it for us, Vincent, and for our baby." Eleanor tells Millett that she had to kill all the women that he lusted after because she loved him. She talks about an accepted ritual within a society where women are not allowed to defend themselves. When they both hear police sirens outside their house, Eleanor says that the police are coming for her and that Judd may know she committed all the beheading murders. Eleanor goes to the front door and prepares to make a run for it. She says that if the police kill her, they will kill her unborn baby too. Eleanor also tells Millett that she always knew that she would get caught someday, and she is going to make a suicidal attempt to flee. Millett stops a weeping Eleanor from going to the door.
Outside, Judd, Taj, and several policemen are outside preparing to move in with guns drawn. Suddenly, a figure wearing the black motorcycle helmet and black leather speeds away from the back of the row house. The cops give chase until it ends with the motorbike hitting a car at a police blockade and the person in black flies off the motorbike and lands on the hood of Taj's car. The helmet is taken off and the cops see Millett. He is dead from either a broken neck or blunt force trauma to his head. Taj declares that Millett is the killer, but Judd does not seem satisfied with the discovery.
A few days later at the professor's funeral, Judd shows up and asks the grieving and distraught Eleanor (who's the only one in attendance) if the ceremony is over. She cries and says 'yes'. (It is clearly implied that Judd correctly suspects Eleanor to be the killer after his encounter with the helmeted killer at Helen's house earlier because of the killer's height and slim build after having interacted with both Eleanor and Millett repeatedly). Aware that Eleanor was indeed the killer but with the case closed of Millett being the culprit, Judd seems to accept it and walks away. Eleanor, aware that Judd knows, walks the other way, alone.
A little later, Judd is in his office at the police station working as usual. He leaves his office and goes to his car in the garage to drive home when suddenly he is attacked by a black helmeted leather-clad figure hiding in the back seat. It turns out to be Taj just wanting to scare him, and the two cops laugh.