I. Synopsis
Kang Seo-jin (Kim Mu-yeol) is an architect who lost his wife nearly six months ago in a hit-and-run accident. Stricken with the grief of not finding the driver responsible, he struggles to manage his work and take care of his daughter.
Seo-jin's daughter Ye-na still doesn't know about her mother's death. Seo jin gets a call from an orphanage that claims that his long-lost sister was found after 25 years, Kang Yoo-jin (Song Ji-hyo). He meets her but neither has clear memories about what happened on the day of her abduction. The family goes to the hospital where Yoo-jin works and meets a fellow nurse, Yeon-joo, who praises Yoo-jin.
Yoo-jin soon wins over her entire family. Seo-jin's parents and daughter start acting differently with him and instead listen to only Yoo-jin. Seo-jin becomes suspicious and wonders if she is really his lost sister. The police inform him about new evidence in his wife's accident. He notices Yoo-jin in the footage, making him even more suspicious. He consults his psychiatrist friend for hypnosis therapy. He sees the driver who hit his wife and his "sister" watching from a distance. He goes to the orphanage but finds that it doesn't exist. He then discovers that Yeon-joo was hired by Yoo-jin to tell her family about her being a nurse. Enraged, he reveals this to his family. Yoo-jin apologizes and says she wanted to create a good impression. Their mother faints and is taken to hospital, where Seo-jin manages to get a strand of Yoo-jin's hair. He takes the sample for DNA analysis.
One night Seo-jin gets a call from a realtor that there is someone in his old home. When he gets there, he finds Yeon-joo's dead body and faints. He wakes up tied by the man who killed his wife. The man tells Seo-jin that he was the one who called him from the "orphanage" and that killing his wife was unintentional. He tells Seo-jin that they have a religious cult where they worship "The Chosen Holy Child." They abduct girls and perform rituals on the child and their believers worship the child. The chosen child now is Ye-na. Seo-jin kills the man and comes home only to find the police. The police and Seo-jin's friend believe that Seo-jin is behaving wildly because of his medication. They arrest him. Yoo-jin confesses to him that she had met his wife and made her join their cult.
Seo-jin flees from the police as Yoo-jin tries to escape with Ye-na. They get into a tussle, culminating in Yoo-jin about to fall off a cliff. She pleads with Seo-jin not to let go but he leaves her to fall after remembering what happened on the day of his sister's abduction, telling her that even if she were his real sister, she could never be their family. The film ends with Seo-jin shredding the DNA analysis he requested without looking at the result.
