In a world where dungeons have appeared and humans have gained magical powers, the amount of magic one is born with determines their strength. Amachi Kai, a weak conqueror with very little magical power, enters dungeons to earn money for the medical treatment of his sick younger sister, Nagi. But when Nagi’s condition suddenly worsens, Kai is faced with an impossible situation—he urgently needs a massive amount of money for her treatment. Driven into a corner, Kai recklessly joins an investigation into an unknown dungeon with an enormous reward at stake.