Right in the middle of Shindo Seitarou’s high school trip an accident occurs. While riding in a bus, his entire class “wanders” into another world. In that world, people like them are known as “lost ones” and are a relatively common occurrence. In the process, Seitarou receives the powers of a character he admires — a dark hero named Black Knight. With his newly acquired strength, Seitarou believes he can also become a hero, just like his idol, however, that might prove to be a much harder task than he expects…