In this day and age, there is a dungeon named “Tengoku” - a huge tower far beyond even the Karman Line.
In Japan, the sword that contains the power of the devil, the “Demon Sword,” is in one's hands, and one's own strength is all that is needed to stand on one's own feet.
While many young people are yearning to become a “demon swordsman,” Jinya Butterfly, who is not particularly interested in such things and is working part-time for a shady employer, happens to acquire a demon sword one day and becomes one of the demon swordsmen.