Ten years ago, gates to another world suddenly opened, where people were allowed to kill monsters. Thus hunters appeared, pursuing and destroying beasts. But not everyone among them was destined to raise their level and become stronger. Son Jin-u was a hunter of the lower E-rank, who had no chance to climb the rank ladder, until one day he accidentally found himself in a D-rank dungeon. Having barely survived the monsters' hands, Jin-u discovered the secret of leveling up.