The story is a locked-room mystery where 12 teenagers go into an abandoned hospital to give up on life. In the room where they meet, a boy is already lying dead, and no one knows who he is or who killed him. The boy is named "No. 0" by the group, as he was the first one to arrive, and they attempt to find his murderer. During their search, the reasons why they want to die are revealed one by one.