In the year 1999, the feared Great King of Terror did not descend—but the world instead fused with another world. Magic, non-human races, and dungeons began to appear across the Earth. Humanity endured the many upheavals that followed this “World Fusion,” and, by making use of the resources brought back by dungeon explorers, people settled into a new way of life—and a quarter of a century passed.

Dungeon exploration streams became a popular form of entertainment, and explorers rose to become the most sought-after profession in the world. Delving into dungeons, they returned a variety of resources to society and played a vital role in suppressing “Dungeon Rampages” by conquering the dungeons themselves. Meritocratic explorer clans, all-female clans with idol-like popularity, and stateless clans that roamed the globe all flourished… and within this society, Hayasaka Tooru worked as a cleaner in his local dungeon.

Those deemed to have no aptitude for dungeons could not become explorers. However, leaving unpopular dungeons unattended increased the risk of Dungeon Rampages, so cleaners were required to hunt low-level monsters in the upper floors of municipally operated D-rank dungeons. Tooru had been working as one of these cleaners for about three years, spending his days endlessly grinding weak monsters on the upper levels of a D-rank dungeon.

Then one day, through a twist of fate, Tooru encountered an anomaly within the dungeon and stepped on a teleport trap. What awaited him was an area crawling with powerful monsters that could never have existed on the upper floors of a D-rank dungeon. Fleeing for his life, he stumbled into a hidden room—and there, embedded in the floor, he found a mythical-class sword.

Not just one, but two.

This is the story of a bottom-tier cleaner who, in a modern dungeon, ended up obtaining both a holy sword and a cursed blade.