In rural Pennsylvania, a young girl screams for help in a meadow, her leg caught in an animal trap. A group of children, along with an older woman they call Mama, arrive, and one of the boys, Abraham, John Amplas knocks the girl unconscious. Later, the children chant a Satanic prayer over the girl at the behest of their mother, and one of the children, Cynthia, proceeds to brutally stab her to death with a dagger.
Years later, teenaged Nancy Johnson, Melanie Verlin, runs away from her home after her stepfather Bert Johnson, Lawrence Tierney, a police officer, attempts to rape her. While hitchhiking to her sister's home in San Francisco, she is picked up by two benevolent young men, Hank and Tom. In the backwoods, they pick up a Baptist preacher and his adult daughter, Sandra, who they drop off at a cemetery to visit the grave of the preacher's wife. Sandra says a prayer and heads back toward their house; her father remains, and is stabbed to death by a man with a machete. Later, Sandra finds her father's corpse on their doorstep, and is also murdered by the man with the machete.
At a local bar, Tom, Hank, and Nancy encounter racists who refuse to serve them because Hank is African-American. Short on money, they steal groceries from a small market and are chased by two local police officers. They lose the police by driving onto a dirt road into the woods. Upon stopping, they witness a man in the woods carrying a large object draped with a sheet. Not wanting to be caught by police, they ultimately decide to camp in the woods overnight.
In the morning, Nancy goes for a walk, and returns to the campsite to witness two police officers, Luke and Abraham, arresting Hank and Tom, accusing them of murdering a local woman. The police shoot Hank and Tom to death execution style, and then pursue Nancy, who flees into the woods. She comes across a farmhouse, and inside finds a teenage girl, Cynthia, playing cards. She asks for a phone, and is directed to another room; when she enters, she finds the man she, Tom, and Hank saw earlier (whom she comes to find is named Cyrus) dismembering two corpses. She is confronted by the two officers, who lock her in an animal cage next to another victim, Gwen. In conversation, Gwen recounts how Luke and Abraham murdered two police officers the night before and stole their uniforms. Luke, Abraham, and Cyrus return to the campsite and burn the bodies of Hank and Tom. Later, Luke goes upstairs and has a conversation with his dead mother, Harriet Johnson, a decomposed corpse the family keeps in a bed.
Meanwhile, Bert reports Nancy missing and begins searching for her himself. At the farmhouse, the family conduct a Black Mass at midnight and sacrifice Sharon, another local woman they have kidnapped, in the name of Satan. Observing the mass from a cage, Nancy prays to God as they slit Sharon's throat before feeding her blood to their dead mother Harriet Johnson. Luke and Abraham drive Sharon's body to a field the next morning and begin digging a shallow grave, an event witnessed by Bert, who stumbles upon the scene; he hears them discuss sacrificing Nancy and Gwen on Easter.
At their next Black Mass, Cynthia sacrifices Gwen, while Nancy quietly recites the Lord's Prayer. This time, Luke drinks Gwen's blood himself. After the mass, Luke prepares to bring Gwen's body outside, but is accosted by Bert, who clobbers him. Bert holds Abraham and Cyrus at gunpoint, and forces Abraham to retrieve Nancy. Just as Nancy is brought outside, Cynthia attacks Bert, stabbing him to death. As he collapses, his gun discharges, shooting and killing both Cyrus and Abraham. Nancy flees into a barn and is chased by Cynthia, but Nancy manages to overpower her and slash her throat with a sickle. Nancy proceeds to pour gasoline on Luke, who has regained consciousness, and lights him on fire. Traumatized, she collapses against a shed and watches Luke burn to death.
