A high-school girl names Kana suddenly commits suicide in a classroom one afternoon. Her friend, Anzu who at the time hears Kana humming a strange song, is on hand to witness the tragic scene, And While the disturbing event leads to media speculation about bullying at the school, Anzu cannot shake the feeling that something more might be going on. With the mysterious melody still fresh in her mind, she decides to investigate the mystery with her friends...