A high school boy named Keisuke Sakudaira is suddenly summoned to another world just before he enters high school, where he's told he must defeat the Demon King as a hero. However, the sacred weapon (Holy Armament) bestowed upon him is nothing more than his ordinary school bag...!? Soon after, Keisuke is labeled as a defective hero and is abandoned by the queen, his summoner, in the most dangerous dungeon filled with powerful monsters. It seems he is doomed to die... But then, the "useless weapon" — the school bag — is revealed to have amazing cheat abilities that will change everything!