A broken suicide pact leads to a malevolent mystery at a Catholic girls' school in South Korea. When Jung-un discovers her older sister Unjoo has taken her own, she's devastated. Convinced that her sister wouldn't commit suicide, and disturbed by rumors making the rounds at school, Jung-un begins a comprehensive investigation into Unjoo's untimely death. Just before Unjoo died, she made a suicide pact with fellow students Soy, Eugene, and Eun-young. So why are the other girls still alive? The closer Jung-un gets to the truth, the more she realizes just how vicious her manipulative classmates can truly be.