On a remote tropical island, a devoted Christian couple works as missionaries, guiding the local community as a new church rises under their watch. The partnership between faith and tradition is tested by the islanders’ enduring Pagan-like rituals, and the tension peaks when their young daughter, Madeleine, becomes drawn into those local ceremonies. The clash of beliefs ultimately erupts in tragedy as the church is set ablaze, leaving a stain of guilt and unanswered questions in its wake.
Several years later, the story shifts to the present life of Toby Burns, a high-school senior who is already grappling with trauma after his father is killed by carjackers. His therapist prescribes a behavioral rehabilitation program designed to redirect his energy and emotions. He and a group of other participants are ferried to the same island for a multi-day ordeal, each of them dropped off in a different corner of the landscape. Their supervisor, Kay, promises to return in three days to collect them, setting the stage for a solitary, survival-driven challenge that will test every participant’s limits.
From the outset, Toby encounters one obstacle after another. Monkeys raid his food, making it hard to keep his spirits up, and a discarded tribal mask in the sea keeps him from moving freely. He struggles to kindle a fire and contends with dehydration and seasick-like symptoms, until a sudden collapse forces him to confront how alone he is on the wild shore. Awakening the next day, he discovers a lifeline in a stranger who appears from the trees: a sixteen-year-old girl named Madeleine, an adept survivor who quickly teaches him the raw, practical skills he needs to endure.
As their bond grows, he shares stories of a world beyond the island’s coast, while she reveals how much she has missed by never leaving. The moment they begin to establish a routine, Madeleine asserts her independence one night, revealing that she doesn’t want to be found and warning him not to follow her into the jungle, lest her mother harm him. Kay finally makes landfall to retrieve Toby, but the moment she steps ashore a mysterious force attacks her in the undergrowth. The following morning, Toby discovers Kay’s boat wrecked on the shore, and a woman scavenging the vessel for supplies—only to realize the woman is Madeleine’s mother, a figure who radiates danger and sorrow.
Toby follows the trail into the jungle and confronts the truth about the two women. Madeleine and her mother return just as Toby traces the remnants of the missionaries’ settlement, the scene of past losses and present fears. Madeleine urges him to head toward the island’s eastern coast for a future meetup, while she recounts glimpses of her life before the tragedy that fractured the family. Her mother, or whatever spectral force has haunted them all, has been pursuing them—hiding a more terrifying secret: Madeleine herself has carried a dark, monster-like power inside her since childhood.
The film then unspools a chilling, unfinished backstory. When Madeleine was young, her parents confronted the locals amid a sacred rite that involved animal sacrifices. During the confrontation, an unattended Madeleine wandered into a ritual sacrifice and was possessed by a malevolent spirit. Transformed into a feral, demon-like creature, she slaughtered those around her, sparing only her mother. Since that day, her mother has kept Madeleine secluded on the island to protect the outside world from her danger. In a desperate moment, she administers berry juice to calm Madeleine, then dies from her injuries, passing the burden to Toby in the immediate aftermath.
Back at camp, Toby finds Madeleine returning to her feral state and dragging a dead Cameron through the forest. He witnesses her return to herself but notes she cannot recall the carnage she caused. With her mother now dead, Madeleine seems poised to leave the island for good, rejoicing at the prospect of normal life. However, her attempt to draw closer to Toby is met with resistance; he subdues her, forcing berry juice down her throat and binding her to prevent further violence. He then reclaims a functioning lighthouse, realizing it could serve as a beacon for help. His plan hinges on a nearby patrol boat catching wind of the distress signal.
As the coast watchers approach, the confrontation with Madeleine intensifies. The feral version of she returns, pushing Toby to his limits as he fights to keep her at bay long enough to ignite a fire and keep the lighthouse’s signal alive. He succeeds in luring the vessel closer, but Madeleine remains a relentless threat, closing in with a mix of calculation and primal power. In the end, the rescue boat’s captain requests confirmation that anyone else still remains on the island, only to be met by the sight of a figure swathed in a blanket—seemingly Toby—but the film’s final twist reveals that the person aboard is Madeleine, leaving the island as unsettled as ever and hinting at an unresolved cycle of danger that continues to haunt the shore.
