In August 1975, at Camp Emerson in the Adirondacks, counselor Louise Donnadieu wakes up to discover that Barbara Van Laar, a privileged but troubled camper, is missing. A chaotic search begins, involving the camp’s director, T.J. Hewitt, the Van Laar family, and local authorities. Barbara’s disappearance echoes a past tragedy: fourteen years earlier, her eight-year-old brother Bear vanished from the family’s estate, Self-Reliance, and was never found. Self-Reliance sits on the Van Laar Preserve, only half a mile north of Camp Emerson, and both properties are owned by Peter Van Laar II.  

The night Barbara goes missing, both Louise and her counselor-in-training, Annabel, sneak out at different times—Annabel to drink with a boy, and Louise to meet up with Lee Towson, the camp’s prep cook. Louise is caught flirting with Lee by her fiancé, John Paul McLellan Jr., which leads to a fight between the two men. 

Two months earlier, Barbara arrives at Camp Emerson as an edgy outsider, drawing curiosity and admiration. She befriends Tracy, a lonely girl struggling to fit in, and confides in her about sneaking out at night. Alice Van Laar, Barbara’s mother, sees camp as a convenient way to send Barbara away for the summer before placing her in a strict reform school in the fall. Alice is emotionally distant and struggles with abuse of pills and alcohol, a result of her devastation over Bear’s disappearance. 

The story flashes back to Alice meeting her husband, Peter Van Laar III, in the 1950s, quickly marrying into his powerful family. Peter’s behavior toward Alice grows increasingly controlling and dismissive. He belittles her intelligence, often undermining her, and demands that she meet his standards of perfection. Alice heavily relies on drinking to meet his expectations, and she often reflects on how she has never felt autonomous in her life. Later, in 1961, Bear is hiking with his grandfather, Peter Van Laar II, but when he leaves his grandfather and goes back to retrieve his pocket knife, he mysteriously disappears Despite an exhaustive search led by Carl Stoddard, a firefighter and Van Laar estate worker, Bear is never found. His disappearance remains a mystery for decades, explaining the Van Laars' obsessive need for control and secrecy. 

Back in 1975, Investigators Judyta Luptack and Denny Hayes arrive at the camp to investigate Barbara’s case, suspecting the Van Laars are withholding information. Meanwhile, Jacob Sluiter, a convicted murderer and expert survivalist, has recently escaped from prison and is believed to be in the area, leading many to wonder if Barbara is his latest victim. As the search intensifies, Tracy also goes missing temporarily, secretly heading into the woods to look for Barbara. At camp, Louise is falsely accused of drug possession and arrested, while Tracy, lost in the woods, spots a figure who leads her toward safety. 

Judyta continues her work on Barbara’s case, pushing back against sexist remarks from her male colleagues. Peter insists that only Captain LaRochelle, the investigator from Bear’s case, will handle communications for his daughter’s case. But as Judyta investigates, she uncovers disturbing truths about the Van Laar family and Bear’s disappearance. She finds evidence suggesting that the family covered up Bear’s death, falsely blaming Carl Stoddard, who died in police custody. She also learns that Barbara has been sneaking out at night, possibly to meet John Paul McLellan Jr., whose lawyer father has protected him from facing consequences for his degenerate behavior. In a winter flashback, Louise is abused by John Paul and takes refuge at T.J.'s cabin, where she learns about a remote cabin the Hewitts have up north. 

Another flashback to 1961 exposes the truth behind Bear’s disappearance. On the final night of the Blackfly Good-By summer party, Alice is intoxicated and emotional from finding out about her husband’s affair with her sister, Delphine. Despite the approaching storm, she offers to take Bear out onto the lake. The boat capsizes, and Bear drowns. His grandfather, Peter II, covers it up, asking Vic Hewitt, the camp director, to bury Bear’s body in the woods. Alice is sedated and never learns the truth of what happened to her son. 

In 1975, Judyta’s investigation leads her to Vic Hewitt, whom she finds hiding in a room above the slaughterhouse. He is ill and has started to lose his memory. Under questioning, Vic and T.J. confess what they know—that Bear drowned, and the Van Laars orchestrated the cover-up to protect the family’s reputation. After being released from custody, Louise shares with Judyta her knowledge of the Hewitt’s remote cabin.  

As Judyta digs deeper, she learns that Annabel has been seeing John Paul all summer, and that  T.J. Hewitt planted Barbara’s bloody clothes in John Paul’s car to throw off investigators. Judyta discovers that Barbara, feeling abandoned by her family, had long planned to escape her life. T.J., who has acted as Barbara’s caretaker over the years, secretly helps her flee to the cabin, providing her with supplies and survival training. 

In the final days of the investigation, Judyta follows a hunch and swims to the island, where she finds Barbara living in isolation. The two briefly exchange words—Barbara is alive, self-sufficient, and determined to remain hidden. Recognizing Barbara’s happiness, Judyta respects her decision and leaves the island without forcing her to return.