A pair of young people are making out in a car in a dark area next to a large empty house. The girl says she's nervous making out there because the house has an evil reputation, but the boy convinces her there's nothing to worry about. Something resembling a corpse thuds against their car and they speed off, but the scare was a joke perpetrated by two young men who are lurking in the area. One is inside the house and the other is by the road, and they call out to one another, laughing about their joke. Then something inside the house manifests itself to the boy inside; doors start closing, and a dark presence stalks him until he's hurled against the ceiling, killing him. His friend comes inside to find him and is also stalked by the presence. He tries to escape through a window, but the window slams shut on him and holds him still while something appears behind him and slashes his lower body until his body is completely cut in half. His upper body falls outside the window while his legs fall inside the house.
Inspector Sturgis pays a visit to Reverend David Thompson, a new clergyman assigned to the parish who is taking over for an elderly minister named Reverend Maier. Sturgis complains about the house, which is on property owned by the church. The two boys murdered there have brought the safety concerns of the house to the forefront, as well as the small body of water on the property, known as Black Pond. Sturgis says local teens sometimes go there to skinny dip at midnight, and there have been several drownings. Thompson tells him that the property will no longer be a concern, since there is a new priest coming to the parish who will be living there with his family, Reverend George Leahy. He and Maier go to the house and meet Arlen, the animalistic caretaker of the property grounds, who lives in a small bungalow with his aging mother. The police suspect Arlen of the two murders but are unable to establish a case against him just yet. Sturgis tells his partner, Hollister, to follow Arlen, who goes off running in the direction of the pond. Hollister investigates the dock where Arlen was standing, and is suddenly pulled under the water by a blackened, clawed hand that emerges from the pond. When Hollister doesn't return, Sturgis holds Arlen and has the pond dragged. Nobody is able to turn up his body, and Thompson announces he's going to have the pond completely drained. Arlen reacts violently, breaking free from the police and running off into the woods. Maier and Thompson talk to his mother, Elvira, who tells them her son serves a secret "mistress", something the two men write off as fantasy. Back at the house, Thompson meets a friendly young girl named Mary who seems to appear out of nowhere and vanishes just the same way. Before the ministers can bless the house, a freak accident occurs when the blade of a table saw being used suddenly flies off the machine and embeds itself into Rev. Maier, maintaining its rotation until it has sawed completely through his body.
The Leahys move into the house anyway: Reverend George along with this wife, Melinda, and their three children: teenage girls Ann and Sheryl, and preteen Justin. They are apparently undeterred by the fact that so many people have died in the last week in their new home...not to mention the fact that the last tenants of the house, back in the 1950s, were found murdered in one way or another. On the day of their move-in, one of the renovators is killed by the black-clawed figure when he goes off into the basement alone to turn off the power in the house's old elevator. His disappearance goes unnoticed by the rest of the crew, and they leave without him. Justin Leahy, the youngest of Reverend Leahy's three children, hears the commotion in the basement and investigates, when suddenly the little girl Mary appears and introduces herself. She runs off before Justin can find out who she is. Justin excitedly tells the others that they have their own "pool"...the pond. They all go down to the pond to relax, where Ann jumps in to swim, but she suddenly starts screaming. When they drag her out of the pond, she has a severed human hand clamped around her ankle, presumably that of Detective Hollister. As Ann lays in her bed, languishing from shock, Melinda tells George they simply can't move out of the house.
Sturgis still thinks Arlen is responsible for the murders, but Elvira insists that Arlen is just protecting his "mistress", cleaning up her 'leavings' and guarding them against her. She is dismissed as insane. Meanwhile, after Justin finds George drinking alcohol in the basement of the house, he returns there on his own and is killed by the witch when he discovers the body she left in the elevator shaft. A search is mounted for him, but Sturgis tells the rest of the Leahy family to stay out of it and leave it to him and his men, along with Thompson. Ann has a nightmare of the family being slaughtered. When she later tries to comfort the worrying Melinda, Sheryl gets sassy and mocks their father, whom she considers to be weak and ineffectual due to his alcoholism. Ann slaps her and says "Shut your bitchy mouth!"
Thompson leaves to researches the folklore that Elvira has told them about Arlen's 'mistress', but before he leaves, Elvira tells him that the antique crucifix he has recovered from the pond was keeping the witch dormant until nightfall. Now that the cross has been removed, she can move about in the early hours of the morning. Thompson remains unconvinced, and is fixated on finding Arlen. Later, Thompson discovers a book in the church archives documenting events of the Inquisition. It seems a woman was executed for being a witch in that location in 1692, and a flashback sequence depicts it. Elondra Sharack is accused of witchcraft by a nine year old girl named Mary (the same one that's been appearing to people in present times). It seems Mary is correct, as Elondra is shackled on a plank and constantly affirms Satan as her master and insists she will take revenge on the pious-looking Reverend Pike, who condemns her to death by drowning. When the villagers execute Elondra, her voice deepens into a demonic howl and her face morphs in a bizarre fashion. The moment they drown her, the church erupts in flames and her laughter is heard everywhere. The flashback establishes that Elondra continued to haunt the area after her death, resulting in numerous deaths and disappearances. Left alone is Reverend Pike, whom she murders by crushing him in a wine press.
Sturgis and his men discover a secret room off the house's basement, where Arlen has been hiding. They also find the body of the elevator repairman. The police take Arlen away in a squad car, while Sturgis stays behind to investigate the secret room, where he is killed by the witch. Melinda gets restless with waiting for some news about Justin, and she ventures downstairs. The witch emerges from the basement, trapping Melinda in the kitchen and hurling her around the room until she is bloodied and beaten. The girls are terrified by her screams and George attempts to intervene. Thompson returns, armed with the knowledge of the witch and still carrying the crucifix, which seems to undo her evil magic momentarily by allowing him to enter the supernaturally locked doors of the house. They are able to enter the kitchen, but Melinda is nowhere to be found. Thompson tells George to go back upstairs, where he barricades himself in his bedroom and discovers Melinda's dead body in their bed. The witch appears in his window and causes his death when a mirror explodes and slashes his face and throat. Thompson tries to save him but is too late.
He attempts to get the girls out of the house but Sheryl runs off on her own into the attic, where the witch attacks her and gruesomely murders her by nailing her head to the floor with a large spike. Thompson sends Ann running from the house and gives her the keys to his van, telling her to save herself. When he goes into the attic to rescue Sheryl, he finds her body and is attacked by the witch. The crucifix keeps her from touching him, and before she can kill him, she vanishes. Thompson goes outside and realizes the van is still there, and he discovers Ann's dead body in the van. Despondent, he takes cans of gasoline down to the pond and purifies it with fire. Mary appears to him and reveals herself to be simply another form of the witch; Thompson drives the crucifix through her heart and she falls backwards into the pond, shrieking "I cannot die! You cannot kill me!" The disturbance seems over, but before Thompson can leave, Elondra's hand erupts from the pond and pulls him to his death under the water.