| Director | Dario Argento |
|---|---|
| Budget | $3.8 million |
After missing a bus in the Zurich countryside, 14-year-old Danish tourist Vera Brandt comes across a cottage and is beheaded by an unseen assailant, who retains her body.
Eight months later, her case is being inspected by forensic entomologist John McGregor and Inspector Rudolf Geiger of the Kantonspolizei Zürich, who note that Vera is the first in a series of murders of young girls, including McGregor's former assistant. Meanwhile, Jennifer Corvino, the daughter of a famous American actor, arrives at the Swiss Richard Wagner Academy for Girls, chaperoned by Frau Brückner, who places her with roommate Sophie.
At night, Jennifer sleepwalks through the academy and witnesses a student being murdered. She flees into the woods, where McGregor's chimpanzee, Inga, finds her and leads her to him. Upon noticing the affection his captive insects have for Jennifer, McGregor comes to believe that she has a telepathic link with them. The following night, the murderer kills Sophie. Jennifer sleepwalks again and is led by a firefly to the scene of Sophie's murder, where she finds a maggot-infested glove. McGregor identifies the maggots as the larvae of Great Sarcophagus flies, which are drawn to decaying human flesh. He theorises that the killer is a necrophile who has been keeping his victims close to him post-mortem.
Later, when the other students taunt Jennifer for her connection to insects, she summons a swarm of flies that covers the entire building, prompting the headmistress to arrange for her transfer to a mental hospital. Jennifer flees to McGregor's home, where he gives her a glass case with a Great Sarcophagus fly to help her find the murderer's lair. The fly leads her to the same cottage Vera had found, but Jennifer finds it unoccupied. Inspector Geiger, who was secretly following Jennifer, talks to the local real estate agent in order to learn the identity of the house's previous occupant. That night, McGregor is murdered in his home in front of Inga.
Jennifer calls her father's lawyer, Morris Shapiro, to arrange for her return to the United States. He alerts Brückner, who offers to let the girl stay at her house overnight. Meanwhile, Geiger's investigation leads him to a mental hospital in Basel, where he learns that a former staff member had been attacked 15 years before by one of the male inmates.
At Brückner's house, Jennifer notices that all the mirrors are covered; Brückner explains that her son cannot stand to see his own reflection. Brückner insists that Jennifer take pills before she goes to bed; when she does so, she becomes sick and, assuming that the pills are poisonous, coughs them up. She attempts to call Morris, but is knocked unconscious by Brückner. Geiger arrives and questions Brückner, who confirms that she had worked at the Basel mental hospital and had been raped. She leads him to the basement and imprisons him.
After waking, Jennifer engineers her escape through the basement. There, she finds Geiger and falls into a pool filled with maggot-infested corpses. Brückner arrives and taunts Jennifer, but Geiger frees himself and beats Brückner while Jennifer escapes. Jennifer finds Brückner's son Patau, the result of the rape by the inmate, who has a hideously deformed face. He chases Jennifer onto a motorboat and tries to kill her with a spear, which perforates the boat's fuel tank and causes a leak. Jennifer summons a swarm of flies that attack Patau, causing him to fall into the water. Jennifer is forced to jump into the water as the leaking petrol ignites, whereupon Patau grabs her, but he is immolated by the flames.
Jennifer reaches the shore just as Morris arrives. A severely injured Brückner decapitates him from behind with a metal sheet and confesses that she murdered McGregor and Geiger to protect her son. Before she can murder Jennifer, Inga appears and kills Brückner with a straight razor in retribution for McGregor's murder. With the ordeal over, Jennifer and Inga embrace.