It is one of a number of South Korean horror films set in high school; the trend began with 1998's Whispering Corridors.

[1] 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 (Lee Yoon-ji), her sister, Eun-seo (Jeon Hye-bin), scolds the girls.

The amnesiac Min Ji-won (Kim Ha-neul), who is an expert swimmer, begins to have nightmares of a ghost from her forgotten past, which seems to be connected to water.

Ji-won slowly remembers that she used to lead an elitist clique back in high school which consisted of herself, Eun-seo, Yu-jung, and Mi-kyung.