Teenage best friends Abby Rivers and Gretchen Lang embark on an adventurous sleepover at Margaret Chisholm's lake house, alongside their classmate Glee Tanaka. This seemingly innocent gathering quickly takes a dark turn as they play with a Ouija board and inadvertently conjure an evil spirit. The atmosphere intensifies when Margaret's boyfriend Wallace Stoney joins them, leading the group to share the experience of taking LSD.
While under the influence, Abby and Gretchen explore the eerie abandoned building nearby, only to encounter several chilling events, culminating in a frightening attack on Gretchen by a supernatural force. Observing the peculiar change in Gretchen's behavior, Abby becomes increasingly alarmed, especially when she catches the eye of Christian Lemon, a fitness influencer who seems disturbed by Gretchen’s state.
As Gretchen spirals further out of control—exhibiting odd behaviors such as projectile vomiting on Wallace and indiscretely using the restroom—she confides in Abby about her traumatic experience at the lake house, claiming a persistent haunting. Abby, interpreting this as a sexual assault, seeks help from Gretchen's parents and school authorities like Sister Kathleen and Brother Morgan, but her concerns are dismissed, leaving her feeling powerless.
Determined to uncover the truth, Abby returns to the haunted house and discovers the name Andras inscribed on a wall. This revelation puts her at odds with her friends—Glee and Margaret—as her accusations against Wallace lead to tension and violence, including a physical confrontation with him. Gretchen, feeling betrayed, distances herself from Abby, leading to a public humiliation that culminates in Abby severing their friendship.
Despite the estrangement, Gretchen's unsettling transformation into a more popular and manipulative figure causes chaos among their peers. She preys upon Margaret’s insecurities and initiates schemes that threaten Glee’s health, even going so far as to use poisons against them. Meanwhile, Abby tries to unveil the truth about Gretchen’s possession, finding more clues in her belongings.
An alarming visit to Margaret reveals how dangerously ill she has become due to Gretchen’s actions. Desperate for a solution, Abby seeks aid from Christian, who acknowledges that Gretchen is indeed possessed and offers to perform an exorcism.
In a climactic confrontation, Abby leads Gretchen to the lake house where the exorcism is initiated. However, when the demon materializes as Christian’s mother, he panics and flees. With fierce determination, Abby resorts to stabbing Gretchen in an attempt to banish the demon. A battle ensues as Abby carries out a makeshift exorcism, ultimately resulting in the destruction of the malevolent entity.
Weeks later, even though Gretchen relocates to start anew, her friendship with Abby remains untouched, as they continue their annual trips back to the lake house—sharing the bonds of friendship forged through such harrowing experiences.
