[4] The plot involves a Massachusetts family under financial and emotional strain while their fourteen-year-old daughter, Marjorie Barrett, exhibits signs of severe mental illness.

[8] 23-year-old Meredith "Merry" Barrett recalls the horrific events surrounding her sister Marjorie, which were publicized by her family being featured on The Possession, a sensational reality TV show, fifteen years ago.

Marjorie has been acting bizarrely, blurring the lines between schizophrenia and demonic possession, with psychiatric treatment seeming ineffective.

She behaves erratically, says she hears voices, has violent outbursts and night terrors and privately threatens to hurt her younger sister.

John pushes to abandon medical treatment and enlist the help of his church's priest, Father Wanderly, and comes to believe Marjorie is possessed by a demon.

The family are ostracized by their community, and Sarah comes to disagree with John and Father Wanderly's increasingly cruel behavior toward Marjorie.

For the final episode of The Possession, Marjorie is tied to her bed while Father Wanderly performs an exorcism and her family witnesses.

Merry recalls that Marjorie was injured after her jump, a possible suicide attempt, and John felt abandoned by the church after the failed exorcism.

Marjorie shows Merry dozens of news stories about familicide committed by fathers who, like John, felt disempowered and abandoned by society.

She claims her behavior has been an attempt to draw attention to John's mental issues, but now he is ready to kill the family with Sarah's help.

In both adulthood and childhood Merry is an unreliable narrator, openly admitting that she doesn't clearly remember her past and that she lied as a child.

Prior to these outbursts she is described as a very conscientious and kind sister who often tells her made-up stories based on characters from Richard Scarry's Cars and Trucks and Things That Go.

Karen criticizes how Marjorie was sexualized in the reenactments and dehumanized while she suffered, and how the show's narrative centered on John's plight.

[13] In March 2020, it was reported that STXfilms was to acquire the film, with Scott Cooper set to direct and Margaret Qualley to star.

[14] In February 2024, Veronika Franz and Severin Fiala were announced to write and direct the film, with Fifth Season financing instead of STX.