Tsugumi is a young orphan of mixed European and Japanese heritage who is relentlessly bullied at school in Japan because of his unique red eyes and light hair. Seeking a way out, he accepts an offer from a mysterious benefactor called "Daddy-Long-Legs" to learn English and attend a prestigious boys' boarding school in Europe called Zugvogel Academy.
Upon arriving at the academy, Tsugumi discovers an unusual tradition where the "King" of the school imposes arbitrary rules on the students, which they must follow or risk punishment. To break free from these restraints, one must succeed in the King's cruel and violent game by either hurting others or enduring harm oneself.
Despite his own experiences with bullying, Tsugumi does not want to become a tormentor. After accidentally winning the game on his first day, he seeks a way out. Fortunately, he receives assistance from Licht, the only student to have ever completed all the game's levels. Licht takes a particular interest in Tsugumi for reasons that remain unclear.