After completing his studies at a boarding school, Souji Kushiki, a brilliant scholar and accomplished athlete from an affluent background, comes back home to find his sisters, Harumi, Youko, and Saki, behaving strangely. They display unusual affection and actions that raise concerns, but Souji initially does not pay much attention to it.
Soon after, Souji hears about a tragedy that occurred during the summer break. One of his classmates was pushed onto the tracks in front of a train, leading to their death. In addition, two other students were assaulted by an unknown perpetrator. Souji begins to wonder if his sisters' behavior and the incidents are related and if they have any involvement with the serious crimes.
