At Starliner Towers, a modern high-rise apartment complex on Starliner Island located just outside Montreal, prospective tenants are welcomed by the smarmy manager Merrick. In one of the apartment, Dr. Emil Hobbes, attacks a young woman, named Annabelle, who wears a school uniform. He strangles Annabelle to death, slices open her stomach, and pours acid in before slitting his own throat.
In another apartment, Nicholas Tudor, sits in front of a bathroom mirror, apparently suffering from stomach convulsions. Tudor goes to Annabelle's apartment, and recoils from the scene of carnage, but leaves without telling anyone about it.
The two bodies are found by a security guard who calls the police. The resident doctor, Roger St. Luc is quite calm about it as the paramedics carry the bodies away. St. Luc goes to the office of Hobbes medical partner, Rollo Linsky, who describes the project they were working on which Rollo explains it as: "a parasite that can take over the function of a human organ", and he also reveals Hobbes' penchant for pedophilia, telling St. Luc that he was once caught "examining a young girl's breasts for cancer" when the girl was 12 years old.
Meanwhile, Nick Tudor's neglected and concerned wife, Janine, seeks advice from St. Luc about her husband's sickness and reluctance to see a doctor. At the same time, Tudor leaves work early and goes home to Starliner Towers after more convolutions. At his apartment, he vomits what appears to be feaces over the balcony, just as two old women pass below. It thumps onto one of the women's umbrella, startling them. They think a dead bird landed on them due to the blood-stained splat on the umbrella. The fecal parasite slithers away into the undergrowth.
In the laundry room of the building, an old woman investigates a trail of brown slime leading from an open window to a washing machine. As she lifts the lid, the parasite hurtles out and clamps onto her cheek. She tries to pull it off but cannot.
At Dr. St. Luc's office, an elderly man complains to the doctor about stomach lumps saying that he may have caught them as a sexually transmitted disease from "the girl in 1511" - Annabelle Brown.
Meanwhile, a distressed Janine Tudor visits her friend Betts to confide in her about Nick's recent behavior. Betts tells Janine to give Nick some space while she clearly makes sly remarks to Janine about how she looks. When Janine returns to the apartment, Nick has passed out on the kitchen floor after throwing up blood.
In a corridor, two children play door knocking pranks until a parasite appears in the mail box opener at one of the doors and the kids flee away in terror.
At the Tudor apartment, Nick callously dismisses Janine's worry about his health. When he is alone, he lays on his bed and encourages his chest lumps to move, talking to them as if he is talking to a pet. Meanwhile, Betts runs a bath, listening to a radio announcement about the murder and suicide at Starliner Towers.
In the building physician's office, Dr. St. Luc seems indifferent to the amorous advances from his nurse, Miss Forsythe. They are interrupted by the telephone. Linsky calls St. Luc from Hobbes office downtown where he found Hobbes notes, which far exceed their original project. According to Linsky reading one paper, Hobbes wrote: "Man is an animal who thinks too much", and had a vision about creating a parasite which was "a combination of aphrodisiac and venereal disease that will hopefully turn the world into one mindless orgy". He was also using Annabelle as a guinea pig for his experiment and that he infected with a parasite through sexual contact but when she got out of control he killed her since she could infect others with the parasites. St. Luc tells Linsky that apparently Annabelle was very sexually active and infected three or four men at Starliner Towers and explains about the few cases of stomach lumps already and mentions that Janine Tudor's husband may have been one of Annabelle's sexual partners. Linsky arranges to meet them later in Tudor's apartment to confirm and warns St. Luc about encountering anyone at the building complex behaving in a strange way.
Meanwhile, Betts continues to relax while soaking in her bath, unaware of the parasite forcing its way up through the plughole and heading right for the nearest orifice. Betts screams and struggles as the bathwater turns red and thrashes in agony...
At this point, the chaos really begins as it reaches inexorable ascendancy. Nurse Forsythe stabs a sex-crazed man who jumps her in the building's underground garage. At the same time, a parasite attacks an old French-speaking couple in the corridor before being impaled on the end of the old lady's walking frame. Soon, more assault by residents on others become more frequent. The old lady from the laundry room attacks a delivery man who shows up near her door. The infected delivery man then attacks a young woman and her little daughter, and so on...
Nick attempts to seduce Janine, but she resists when she feels the lumps moving in his stomach. At the same time, St. Luc finds the old French couple who tell him about their encounter with the parasite. The doctor the searches the basement dumpster where the old man claims to have threw the dead parasite, but gets ambushed by another infected man. Dr. St. Luc overpowers his assailant and unhesitatingly beats him to death.
Afraid over what is happening to Nick, Janine is standing by her man as he sleeps... until a bloody parasite emerges from his mouth onto the pillow beside her. In a state of shock, she runs out of the apartment and into Betts... whom is also infected.
St. Luc goes to Merrick's office and tells him to call the police. By now a zombie-like mob of people infected by the parasites is roaming all the corridors of Starliner Towers. The mob breaks into the old French couple's apartment and apparently infects them as well.
In the basement garage, St. Luc finds Nurse Forsythe about to be raped by an infected man, and he shoots the man in the back, killing him with a gun that he took from the office. After attempting to escape from the car park, they are rammed by one of the 'crazies'. They try to get out on foot, but find the garage door closed and locked, and several 'crazies' approaching them. They are forced to flee back into the building. St. Luc and the injured Nurse Forsythe hide in the basement, waiting for the police to show up.
In the lobby of the building, a young couple complains to Merrick (clearly infected at this point) about the disturbances. He ushers them into his office where the couple are immediately pounced on by a delirious group of orgyists and are infected as well.
In Betts apartment, she comforts Janine, stroking her hair and leaning in close. She tells Janine that she always had a crush on her. As the two of them kiss, a parasite exchange takes place and Janine becomes infected.
A little later, Linsky arrives at the Starliner Towers and hears distant noises, but nobody is around the lobby or anywhere else. He goes up to the Tudor apartment and finds the door wide open. Tudor is lying on the bed, his stomach seething with parasites. One of them lunges at Linsky who tries tearing them off. As he tries to remove them by stabbing them with a kitchen knife, he is attacked and beaten to death by Tutor, who tears a parasite off the dead Linsky's face and crams it into his own mouth as an act of ownership.
In the basement, Nurse Forsythe relates a dream she had about an old man saying: "he tells me that even old flesh is erotic flesh, that disease is the love of two alien kinds of creatures for each other, that even dying is an act of eroticism". A parasite appears crawling out of her mouth, and St. Luc knows right away that she is infected. He punchers her unconscious. As he carries her along a corridor, the wooden wall panels burst apart and a sea of arms break through, reaching at them. St. Luc is forced to leave the infected Forsythe behind and runs to Tudor's apartment. He encounters Tudor in the kitchen kneeling over Linsky's bloody corpse. St. Luc shoots Nick Tudor dead. He then tries to escape from the building, but finds all the exits blocked. He encounters a gay couple trying to get at him, then an old man with a young girl in another apartment.
St. Luc makes his way to the indoor swimming pool where he sees the infected Betts and Janine there fooling around. St. Luc finds an open patio door and walks out. Sensing that he is free, he runs away from the pool area and outdoor patio and up a grass river embankment only to be confronted by HUNDREDS of "changed" people slowly and silently advancing towards him. Forced to retreat back into the pool area, he is pushed into the pool where the infected Betts, Janine, and even Nurse Forsythe await. As the mob of 'crazies' jump in and attack him from all directions, St. Luc finally loses his struggle to remain 'human' and allows the mob to consume him.
In the final scene, the time is 5:26 a.m. about an hour before dawn as a radio announcement describes a wave of unexplained sexual assaults across Montreal and the surrounding area which may have originated at Starliner Towers and that the authorities as well as the centers for disease control are currently investigating a possible epidemic of some kind. The final shots over the end credits show a long line of cars with infected people leaving Starliner Island and driving across the bridge leading to Montreal and the rest of the outside world....