In 1985, in the small town of Kinnakee, Kansas, Libby Day is the only survivor of a horrific massacre that claimed the lives of her mother and sisters. When questioned by the police, she points her finger at her brother Ben (Nicholas Hoult), claiming that he is the perpetrator. Fast forward to the present day, and Libby (Charlize Theron) has been relying on donations from sympathetic strangers who remember the little girl from news broadcasts. However, with the flow of money dwindling, she finds herself in a tight financial spot when Lyle Wirth (Nicholas Hoult) invites her to appear at his "True Crime" club for a payment of $700.
During the flashback scenes, we learn that Ben has transformed himself visually by dyeing his hair black, hinting at his troubled state of mind. He struggles with a strained relationship with their mother Patty (Christina Hendricks) and faces ridicule from his older sister, Michelle, who mocks him for being an outsider. Ben's inclination towards nefarious company, particularly a Satanist bookie named Trey Teepano, complicates his life further. Meanwhile, their mother is grappling with the looming threat of foreclosure on the family farm.
As Libby attends Wirth's club, she discovers a surprising twist: many members believe in Ben's innocence, suggesting that her testimony was instrumental in his conviction. Even though she still holds the belief that he is guilty, Libby decides to collaborate with the club for the money she desperately needs. Urged by Wirth, she visits Ben in prison, which sparks a newfound curiosity about the events leading to their family's tragedy.
In the past, Ben had been planning to escape with his pregnant girlfriend, Diondra (Chloë Grace Moretz). He had already been subject to allegations of molestation from several young girls, which threw the family into turmoil. Patty's distress only deepens when she visits Krissi Cates, the chief accuser, who has influenced the other girls against Ben. This visit to Krissi brings Patty face-to-face with a furious parent, heightening her anxiety.
Back at home, their father Runner is a menacing presence, ready to lash out at Patty for money, having incurred debts to Trey. In the present day, Libby learns about Diondra's pregnancy and tracks down Runner in a decrepit factory, where he reveals uncomfortable truths about the past. On her quest for the truth, Libby meets Krissi, who ultimately admits that she fabricated her accusations against Ben.
Meanwhile, in 1985, Patty desperately seeks help for her farm woes and meets a stranger in the dead of night, unaware that he is Calvin Diehl, a serial killer dubbed the Angel of Debt. He targets individuals to collect life insurance, revealing the dark web of deceit surrounding their lives. As the situation spirals, Ben and Diondra hurriedly attempt to flee after hearing about the molestation accusations, leading to chaos and violence at their home.
Libby’s journey culminates in a dire confrontation with Diondra and a revelation that threatens her very safety. As she confronts the realities of her family's past and the lengths Ben went to protect his unborn child, she embarks on a quest for closure. The narrative twists reveal truth and redemption as Libby stands firm, ultimately understanding the choices made by those she once blamed. In a dramatic turn of events, Ben is exonerated, and Libby must decide the fate of Diondra's daughter, caught in the turmoil of a tragedy that has haunted her for decades.
