Kang Seo-jin, an architect, is still reeling from the hit-and-run death of his wife nearly six months earlier. The loss has left him juggling a demanding career and the fragile task of looking after his young daughter, Ye-na, who remains unaware of the true circumstances of her mother's death. In the midst of his grief, a call from an orphanage arrives with a startling claim: his long-lost sister, Kang Yoo-jin, has been found after 25 years. The meeting between Seo-jin and Yoo-jin is strained and filled with memory gaps for both of them, as neither can clearly recall what happened on the day of the abduction that changed their lives.
From the moment Yoo-jin enters the family circle, she exerts an unusual pull. She is kind, attentive, and quickly ingratiates herself with Seo-jin’s parents and his daughter, drawing their attention away from Seo-jin with a seamless ease. The more they lean on Yoo-jin, the more Seo-jin’s suspicions grow. He wonders if Yoo-jin might truly be his sister, or if something more unsettling is at play. Puzzling clues begin to accumulate: new references to the past, a driver who may have been involved in his wife’s death glimpsed in old footage, and a sense that someone is orchestrating a larger game around their family’s vulnerability. In a bid to understand the truth, Seo-jin enlist the help of a psychiatrist friend and even undergoes hypnosis to probe the depths of his memories.
The investigation takes a startling turn when Seo-jin discovers that the orphanage he was told about does not exist. His growing unease deepens into alarm as he learns that Yeon-joo, a hospital nurse, has been introduced into the family as part of Yoo-jin’s plan. Yeon-joo’s role becomes clearer: she was hired by Yoo-jin to help shape the narrative for Yoo-jin’s arrival and to influence the family. In a moment of confrontation, Yoo-jin offers a remorseful apology, explaining that her aim was simply to make a strong impression. The family’s mood darkens, and a medical crisis jolts the household when their mother faints, prompting Seo-jin to obtain a strand of Yoo-jin’s hair for a DNA test to confirm their ties.
A ominous call interrupts the uneasy peace: a realtor informs Seo-jin that someone has been seen at his old house. When he returns to the house, he encounters Yeon-joo’s dead body and collapses, only to awaken to a confrontation with the man who killed his wife. This stranger reveals that he belongs to a religious cult that worships “The Chosen Holy Child,” a cult that abducts girls for ritual purposes and venerates the child they believe is chosen. The chilling implication is that Ye-na has become that child. In a furious and brutal struggle, Seo-jin manages to kill the man and returns home, where authorities and Seo-jin’s friend struggle to judge his behavior as a product of medication or a rational response to trauma. He is arrested, while Yoo-jin confesses to having met his wife and convincing her to join the cult.
Seo-jin escapes custody, while Yoo-jin makes one final bid to flee with Ye-na. A tense chase leads to a cliffside confrontation in which, as Ye-jin clings to life, she pleads with Seo-jin not to let go. He recalls the day of his sister’s disappearance and makes a stark choice: even if Yoo-jin were his real sister, she could never belong to their family. In a devastating moment of resolution, Seo-jin lets go, and Yoo-jin falls away. The film closes with Seo-jin shredding the DNA analysis he had requested, choosing not to look at the results, leaving the truth forever unsettled in the air.
