Kim Moo-yul plays Kang Seo-jin, an architect who lost his wife in a hit-and-run about six months ago, and who struggles to manage his work while taking care of his daughter Ye-na.
Seo-jin soon receives a call from an orphanage claiming his long-lost sister has resurfaced after 25 years, Kang Yoo-jin, Song Ji-hyo. He meets her, but neither of them has clear memories about the day of her abduction, leaving a fog of questions hovering over the reunion. The pair visit the hospital where Yoo-jin works, and there they encounter Yeon-joo, a nurse, Han Hye-ji, who praises Yoo-jin and swears by her kind demeanor.
Yoo-jin quickly wins over Seo-jin’s family, and his parents and daughter begin behaving differently, suddenly listening to Yoo-jin and giving her the authority in the house. This swift loyalty makes Seo-jin increasingly suspicious, and he wonders if Yoo-jin could really be his sister after all. The police deliver news of new evidence in his wife’s accident, and Seo-jin spots Yoo-jin in the CCTV footage, which only deepens his unease. In a bid to unravel the truth, he turns to hypnosis therapy through a psychiatrist friend and finds himself face-to-face with the driver who hit his wife—and with his supposed sister—watching from a distance.
Desperate for answers, Seo-jin traces the orphanage but discovers it does not exist. He uncovers a startling truth: Yeon-joo was hired by Yoo-jin to tell the family about Yoo-jin’s nursing role and to cultivate the illusion of a normal family unit. Enraged, he confronts his family with the revelation. Yoo-jin apologizes, explaining she wanted to make a good impression, but the moment is interrupted when their mother faints and is taken to the hospital. Seo-jin manages to secure a strand of Yoo-jin’s hair to submit for DNA analysis.
One night, a realtor phones with news of someone lurking in Seo-jin’s old home. When he arrives, he finds Yeon-joo’s dead body and collapses. He awakens to find himself tied up by the man who killed his wife, who reveals that he led a cult of believers who worship The Chosen Holy Child and abduct girls for its rituals. The current “chosen” child is Ye-na. Seo-jin kills the man and returns home, where police and his friend insist he’s acting erratically because of his medication, and he is arrested.
Yoo-jin confesses that she had met Seo-jin’s wife and coerced her into joining the cult. Seo-jin then flees as Yoo-jin tries to escape with Ye-na, leading to a tense struggle that ends with Yoo-jin nearly going over a cliff. She pleads with Seo-jin not to let go, but he remembers the day of his sister’s abduction and lets her fall, declaring that even if she were truly his sister, she could never be part of their family. In the final moment, Seo-jin shreds the DNA analysis he had ordered, choosing not to look at the result and leaving the truth unresolved.
