Sam Payne is a 26-year-old woman barely scraping by in Syracuse, New York, doing minimum-wage remote data work after losing her lab-tech job. Her mother, Veronica, has early-onset dementia and lives in an expensive care facility Sam can no longer afford. On the morning of her estranged grandfather Douglas Payne's funeral, Sam opens his final email and finds a link to a podcast about Paynes Hollow, a lakeside community on Lake Ontario where the Payne family has owned over 300 acres for centuries. The podcast questions whether Sam's father, Harris, truly murdered 13-year-old Austin Vandergriff, suggesting that people have vanished near Paynes Hollow for 200 years. Sam dismisses this: At age 12, she caught her father burying Austin's body. Harris died by suicide shortly after, leaving a note about "inner demons." Douglas, convinced Sam was wrong, cut her and Veronica off financially.
At the funeral, Sam reconnects with Sheriff Craig Smits and his daughter Josie from Paynes Hollow. She is pressured into attending the reading of the will, which cannot proceed without her. Lawyer Isabella Jimenez reveals that a developer has offered ten million dollars for the Paynes Hollow property and that Douglas left the entire estate to Sam. The condition: She must spend one full month living in the family cottage. Desperate over her mother's unpaid care bills, Sam agrees.
Sam's aunt Gail, Douglas's younger daughter and a social worker, insists on accompanying her. They find the cottage preserved as a time capsule of 14 years earlier, down to Harris's cardigan on his chair and Sam's childhood drawings on the fridge, all designed to immerse Sam in traumatic memories. Ben Vandergriff, Austin's older brother and the property's caretaker, reveals that Sam must also wear a GPS ankle monitor he will track. Douglas trapped Ben in this role at 16 by tying his father's disability pension to Ben's continued service.
Strange events begin immediately. Sam glimpses a tall figure with dark, liquid eyes in the latched shed and flees. The next morning, a dismembered rabbit appears on the front steps, arranged in a grotesque tableau. The sight triggers a suppressed memory: Austin used to leave similarly mutilated animals in Sam's hiding spots as childhood threats. A second mutilated animal appears days later. Sam also sees lights moving beneath the lake at night and hears distant hoofbeats, echoing her grandfather's stories about a headless horseman of Paynes Hollow.
Tensions escalate when Gail finds a bloodied hatchet in the shed alongside Sam's gardening gloves and concludes Sam is unconsciously staging the incidents to create an excuse to leave. Sam reveals for the first time that Austin tormented her as a child. Late that night, Gail comes to Sam's door saying she can see the lights on the lake, but Sam ignores her. By dawn, Gail is gone. On the beach, Sam finds footprints, signs of a scuffle, and drag marks leading into the lake. Ms. Jimenez confirms Gail's disappearance does not exempt Sam from the will's terms.
Alone that night, Sam sees a figure emerge from the lake: a woman with gray-blue skin, a missing eye, and torn flesh, calling her name. She snaps a blurry photo that unmistakably resembles Gail in a state of decomposition. Searching for answers, Sam finds a book on Great Lakes legends in the cottage's crawl space, including a chapter on nekkers, a Dutch variation of water spirits. Ben then reveals a journal he found hidden in the shed's stone foundation, written by Sam's ancestor. The journal describes how the Payne family created nekkers. A deceased family member and their horse were placed on the lake bottom, and a living Payne, preferably a young child, was bonded to the resulting horseman through a ritual. After 30 days, the horseman emerges to protect the bonded person, killing anyone who physically harms them. To maintain the family's fortune, a human sacrifice must be given to the lake every five years.
Josie, independently researching local disappearances, confirms that over a century, roughly three dozen people who vanished in the region were last seen near Paynes Hollow, aligning with the five-year cycle. Ben connects the dots: Harris likely found Austin's body after the horseman trampled the boy for hurting Sam and was burying it to protect his daughter. Ben confesses he was supposed to watch Austin that night but neglected his duty.
The supernatural threat becomes undeniable when a cyclist who shoves Sam is dragged into the lake by nekkers that night. Both Sam and Ben witness the horseman and the emerging dead. Ben uses the ankle monitor's GPS data to prove Sam never left the cottage during any of the incidents.
Smits takes Ben into the forest under the pretext of a search and returns alone. Sam learns from Ms. Jimenez that Ben's phone died before Smits returned, and that Ben would never allow it because downtime jeopardizes his father's pension. When Sam accuses Smits of grabbing her arm, he recoils in fear, confirming he knows about the nekker bond.
Under pressure, Smits confesses. Josie, who stayed hidden, emerges with a letter from the crawl space revealing an affair between Smits and Veronica and their collaboration in resurrecting the nekker rituals. Smits admits he found the journal before Sam's birth, gave it to Veronica, and together they bonded young Sam to the horseman. Veronica wanted the nekkers' power to stave off the dementia she feared inheriting. When Josie draws her gun demanding to know where Ben is, Smits lunges for the weapon, and the gun fires, killing Josie.
Dropping all pretense, Smits reveals the full truth: Veronica ordered him to murder Harris and forge the suicide note after Sam caught her father burying Austin's body. He has left Ben drugged near the shore as a sacrifice for the nekkers at nightfall. Sam races to the lake as the nekkers emerge. The horseman, Bram, positions himself as her protector. When Smits begins the sacrifice incantation over Ben, Sam takes over the ritual using words she recognizes from the journal and names Smits, not Ben, as the offering. She pulls Ben to safety. The horseman blocks Smits, and the nekkers drag the screaming sheriff into the lake. Gail's nekker form briefly surfaces, showing only fading recognition, before turning back into the water.
Ben wakes hours later and devises a cover story: Smits drugged him, Josie confronted her father, the gun discharged, and Smits fled. Forensic evidence corroborates their account. Ben, who completed law school while working as caretaker, helps Sam navigate the legal process. A life-danger exception allows Sam to inherit without finishing the month.
Sam sells 50 inland acres for nearly two million dollars, pays Ben the $100,000 she promised him, and settles her debts. She visits Veronica during a rare lucid episode and confronts her with everything: the affair, the rituals, the murder of Harris. Veronica deflects and gaslights, confirming the cold nature Sam has finally allowed herself to see. Sam cuts off all contact and financial support. She visits her father's grave for the first time, lays flowers, and apologizes for not believing in him. She leaves on a cross-country road trip with Ben, not as a romantic partner but as a fellow person who needs to heal. Ben drives as Sam watches her father's grave disappear behind them.