A world where dungeons have appeared and humans have acquired magical powers - here, the amount of magical power one is born with is an index of your strength. Amachi Kai, the weakest conqueror with little magical power, is challenging the dungeon to earn money for the medical expenses of his sick sister, Nagi. However, Nagi's condition suddenly worsens. A huge amount of money is urgently needed for her treatment--!? In a dilemma, Kai recklessly participates in the investigation of an unknown dungeon with a large reward at stake...