Teen best friends Abby and Gretchen grapple with an otherworldly demon that takes up residence in Gretchen's body.
The year is 1988. High school sophomores Abby and Gretchen have been best friends since fourth grade. But after an evening of skinny-dipping goes disastrously wrong, Gretchen begins to act...different. She's moody. She's irritable. Bizarre incidents keep happening whenever she's nearby. Abby's investigation leads her to some startling discoveries-and by the time their story reaches its terrifying conclusion, the fate of Abby and Gretchen will be determined by a single question: Is their friendship powerful enough to beat the devil?
Teenage best friends Abby Rivers and Gretchen Lang join their classmates Margaret Chisholm and Glee Tanaka at Margaret's lake house, which is located near an abandoned building where a girl was rumored to have been sacrificed in a satanic ritual, for a sleepover. The four friends inadvertently summon an evil spirit while playing with a Ouija board. Margaret's boyfriend Wallace Stoney then joins the group and everyone drops LSD together.
While tripping on the acid, Abby and Gretchen explore the haunted building nearby. Following several scary experiences, a paranormal presence attacks Gretchen.
Christian, Micah, and Jonah Lemon, who perform as fitness instructors and motivational speakers under the name The Lemon Brothers, put on a show at Aberdale Academy, the Catholic high school attended by Abby and her friends. As Gretchen begins acting oddly, Abby notices Christian Lemon staring concernedly at Gretchen. Abby wonders if the LSD is continuing to adversely affect Gretchen as her behavior continues growing worrisome, including projectile vomiting on Wallace and urinating in front of other students in a classroom.
Gretchen confesses to Abby that she was attacked at the lake house and a presence continues to haunt her. Abby interprets Gretchen's revelation as a sexual assault. Abby speaks to Gretchen's parents as well as Sister Kathleen and Brother Morgan at school, but her concern for Gretchen is threateningly dismissed. Abby returns to the haunted house for clues and finds the name "Andras" scrawled on a wall.
Abby alienates Margaret as well as Glee by suggesting Wallace may have raped Gretchen. Wallace punches Abby in her stomach over the allegation. Gretchen expresses resentment toward Abby for breaking her promise to not tell anyone what happened.
At a school carnival, Gretchen publicly humiliates Abby by submerging her in a dunking booth and revealing Abby's crush on Brother Morgan. Abby tearfully terminates their friendship.
Fully succumbing to her possession, Gretchen suddenly becomes more popular, charismatic, and manipulative at school. Gretchen exploits Margaret's body image issues by poisoning her with diet shakes. In addition to flirting with Wallace, Gretchen also embarrasses Glee by exposing her unrequited crush on Margaret. Gretchen then feeds Glee a brownie with peanuts to induce a severe allergic reaction.
Despite expressing concern for everyone's well-being, her former friends continue to shun Abby. Still hopeful about helping Gretchen, Abby steals her Trapper Keeper to look for more clues on her condition. In addition to drawings of violent imagery, Abby finds the name "Andras" among Gretchen's frightening notes.
Abby visits Margaret, who is now bedridden from emaciation, at her home. Abby discovers Gretchen poisoned Margaret. Long, parasitic worms suddenly emerge from Margaret's mouth.
Abby also visits Glee while she recovers from Gretchen's scheme. After learning what happened to Margaret, Glee implores Abby to stop Gretchen from hurting anyone else.
Remembering the look he previously gave Gretchen, Abby consults Christian Lemon for help. Christian confirms that Gretchen is demonically possessed and recommends that he perform his first solo exorcism on her.
Abby sneaks into Gretchen's house, drugs her drink, and then takes her to the lake house with Christian. Christian begins to exorcise the demon Andras, but Christian abruptly leaves when the demon takes on the appearance of his mother.
Abby stabs Gretchen in her stomach with a fireplace poker after she takes on her full demon form. Abby continues a makeshift exorcism ritual that results in Andras eventually emerging from Gretchen's body. Abby burns the demon to death with fire.
Some time later, Gretchen moves two states away with her parents. However, Abby and Gretchen remain best friends and continue to take an annual trip back to the lake together.
