A Canadian archaeological team in Sicily accidentally unleashes vengeful ghosts of five demonic nuns who were murdered 500 years earlier, and the ghosts now set out to kill the group and townspeople alike.
In southeast Sicily, a team of archaeologists investigate the ruins of a crypt where heretic nuns had been stoned and crucified by the village people in the 16th century. Soon strange things begin to happen, and Liza, one of the team members, starts to have surreal dreams about the nuns. Then the graphic violence begins, starting with a decapitation! The answers lie in the ancient crypt of the nuns.
In 1486, in Sicily, five nuns are crucified in their convent by the superstitious locals that believe they are evil and worship devil. In 1990, in Toronto, the skeptical archaeologist Professor Paul Evans and his superstitious colleague Liza Harris explore with their team the medieval dig where the nuns were killed. Liza has dreadful nightmares with the nuns while members of the expedition mysteriously die and the locals are against the research in the archaeological site.
- Sicily, 1486. A mob of villagers drag five kicking and screaming nuns into a cavern beneath their convent, built atop a steep hillside. They are tortured, nailed to five of the many wooden crosses in the underground chamber.
Toronto, Canada, 1990. A séance is in progress. A young woman named Liza (Meg Register), has a vision of the crucified nuns and falls screaming onto the floor. She is taken home where her boyfriend Professor Paul Evans (Brett Halsey), a noted archaeologist and her former college professor, comforts her.
Several months later. Liza and Professor Evans are surveying Ancient Greek ruins on a site near the small town of Santa Rosalia, Sicily. With them is a team of assistants who are meant to be helping, but spend most of their time drinking. Liza is distracted by the beautiful, but ominous ruin of a monastery which overlooks the dig. Professor Evans chides her for exhibiting a morbid interest in the rumors surrounding the ruins history. The include tales of unspeakable practices conducted there in medieval times. He reminds her that she is supposed to be avoiding any involvement with the supernatural since her mental breakdown back in Canada months earlier.
At the quayside, the professor talks to Porter (Al Cliver), a colleague on his sailboat all about the ruins history in which Porter persuades him to avoid the ruins and concentrate more on the Greek ruins. Also hanging around the dig is Turi De Simone (Lino Salemme), the hot-tempered local butcher, who voices the town's hostility to the strangers. He follows Liza into the ruined building and warns her not to go any further, but she continues after he leaves. In the monastery crypt, Liza becomes convinced that there is another chamber behind a wall and attacks it with a pick-axe. Sure enough, she discovers a cavern containing the charred remains of the five nuns hanging on wooden crosses.
Liza runs outside in horror and bumps into Professor Evans. He seems intent on browbeating her into suppressing all mention of what she found. But the opening of the crypt has apparently freed the spirits of the five nuns, putting into motion a series of supernatural events. That evening, Porter is killed on his boat when he is shot with a harpoon gun by a ghostly nun, who then disappears.
The next day, the people in the village are feeling so irate against the outsiders near their community that the young butcher, Turi, can openly discuss murdering the visitors. Meanwhile, Liza pursues her investgation into the monastery by checking the local church records at the local library, but discovers a huge sheaf of pages missing. A strange-looking woman (Carla Cassola) appears in the dusty corridors and invites Liza to meet her the following day at her apartment to hear the whole story.
That evening, two of the archaeologists, Irishmen Sean (Grady Thomas Clarkson) and Kevin (Pascal Druant), get blind drunk and decide to wander the ruins. They hear giggling female voices and see flittering shadows. Following them, Sean and Kevin are killed when they fall through a weak floorboard and land into a pit of metal spikes. The following morning, Inspector Carter (Lucio Fulci) from Interpol arrives to investigate the deaths, and to question Professor Evans on his knowledge of the area.
Liza meets with the old woman as arranged in her apartment. The old woman tells Liza about what happened at the ruins centuries ago. The nuns practiced witchcraft and held orgies there as well. Local youths wound be invited there for sex, then murdered as they reached orgasm. The nuns would drink their blood in a satanic frenzy. If any one of the crazed nuns would become pregnant, they would carry their unwanted babies to full term, then throw them onto a fire after giving birth to them.
Meanwhile, the police investigation into the dig becomes more urgent when Inspector Carter arrives at the marina looking for Evans' colleague Porter, whose severed head is found impaled on the anchor of his yacht. In town, the old woman is the next victim when she is attacked by her house full of cats and she is clawed to death, her eyes scratched out. When Professor Evans makes a casual remark to Inspector Carter, he puts suspicion on Turi the butcher. But that evening, Turi is killed as he is closing up his shop when an unseen figured dressed in all white attacks him in his walk-in freezer. The figure stabs him with a meat-hook though his neck, and his tongue is nailed to the chopping board. Inspector Carter arrives later that night with the local police to investigate when Turi's wife has called them to her husband's disappearance. Carter finds Turi's dead body locked in the freezer. Carter also finds a piece of torn clothing clutched in Turi's right hand which apparently belonged to his killer.
Suspicion on Turi's murder first falls on Professor Evans who reacts by announcing that the dig will be abandoned, but Liza, acting more and more strangely, refuses to leave.
News of the death of Turi has incensed the townspeople who mount an attack on the haunted ruins. Professor Evans tries to get his team clear, but he can't find Liza, who had wondered off during the night. John (Ettore Comi) and Susie (Christina Engelhardt), a married couple on the team, realize their young son Robby has gone missing too. John runs through a nearby woods looking for Robby, while Professor Evans goes off to look for Liza. Meanwhile, Robby is being dragged through the woods by a white-robed, faceless nun (Liza?). The little boy pulls free and runs through the woods back to camp. Then, Robby inexplicably discovers his father tied by the ankles between two bent saplings. Running to help, the boy trips a rope, the trees spring upright, and John is literally ripped in half down the middle. The blood-splattered Robby returns to the archaeology camp, too traumatized to speak to his screaming and hysterical mother when she finds him covered in blood.
Professor Evans runs towards the ruins looking for Liza who appears dressed in a white robe, obviously processed by the leader of the evil nuns. With a huge mob of townspeople advancing not far behind him, Professor Evans runs into the ruins after Liza who turns around and stabs him in the stomach with a butcher knife, and then disappears like a ghost. The mob of townspeople charge into the ruins, running past Professor Evans lying severely wounded on the ground. The mob converge on the hidden chamber and Liza reappears, foaming at the mouth on one of the crosses. The mob sets her and all the skeletal remains of the nuns afire. The wounded Professor Evans staggers into the cavern, pushing the mob members out of his way, to see Liza materialize at the foot of the burning crosses, no longer processed but dead.