Sasayama Akira was born into a family of assassins and trained as a child to become one of them. However, due to psychological issues, his father decided to send Akira to a relative's house, to live a normal life.

Several years later, Akira gets asked by the student council president to investigate a rumor of a group called Abyss, a shadowy group of students responsible for abducting and killing students in a grotesque game.