Three high school girls are bored and decide to conduct a séance with a bunshinsaba - a traditional Korean ouija board. As the spirit is called by Eun-jung, her sister, Eun-seo, scolds the girls. Eun-jung jokingly wishes that the ghost had taken Eun-seo away and that if they do not send the ghost away, it will kill them. Eun-seo is then killed by a ghostly water presence. The amnesiac Min Ji-won, who is an expert swimmer, begins to have nightmares of a ghost from her forgotten past, which seems to be connected to water. She is confronted by an estranged friend, Yu-jung, who tells her that because of Ji-won, "Su-in" is coming for them. Yu-jung is soon found dead. Ji-won slowly remembers that she used to lead an elitistclique back in high school which consisted of herself, Eun-seo, Yu-jung, and Mi-kyung. Before she became part of the clique, Ji-won was friends with Su-in, but her ego turned her to regard the introverted Su-in as a laughingstock. Ji-won visits her last remaining clique member, Mi-kyung, who is confined to a mental hospital, but the latter lashes out at her. Ji-won learns from Su-in's mother that the event that led to her amnesia was a trip to a forest with the girls, from which Su-in never came back. Mi-kyung is drowned by the ghost. Heading to a spring in the forest, Ji-won finally remembers the event: Ji-won asks Su-in if she can swim, then pushes her into the spring. Su-in comes up and the girls laugh. The three girls then push Ji-won in as a joke, but upon learning that she can't swim, Su-in dived to save her. While Ji-won was saved, Su-in got stuck in the riverbed and drowned. The other girls just watch as this happens. Ji-won alerts the authorities and informs Su-in's grieving mother. Ji-won returns to her mother, only to realize the horrifying truth: Ji-won and Su-in had swapped bodies during their struggle to get out of the spring. "Ji-won" did not lose her memories because she was never Ji-won in the first place; she is actually Su-in. Ji-won's spirit has been terrorizing her and the clique to gain her body back, and now she inhabits her mother. Her mother vomits water and collapses. Ji-won crawls out but Su-in cuts her wrist so Ji-won can't take her. Ji-won's boyfriend, Jun-ho, comes to save them and they are rushed to the hospital. Su-in is later seen walking through the fish market where her mother, who believes her daughter has died, works. She leaves after hesitating, while her mother looks ominously at her with Ji-wom's tell-tale smirk, implying that Ji-won has possessed Su-in's mother.
