The story begins with the protagonist, Mitsurugi Ken'ya, reflecting on his past failures and renewed determination to become a Dungeon Explorer. He was once expelled from a guild for being deemed incompetent, but driven by the need to support his family and his own dreams, he decides to challenge the world of dungeons once more.
The narrative then shifts to a television program where the leader of the top guild "Yoi no Hoshi," Tendou Ryunosuke, discusses his recent achievement of reaching the 59th floor of the Dungeon, a colossal tower that appeared in Tokyo Bay twenty years prior. This event, which resulted in the death of a friend, highlights the inherent dangers of dungeon exploration. The Dungeon is revealed to be a source of "equipment" that grants supernatural abilities, revolutionizing global industries and military power, with Japan holding a significant advantage due to its unique access.
Ken'ya, a high school student, finds himself in a dire financial situation. His mother is ill, and the family has been forced to sell their home and possessions, living in a cramped apartment. His younger sister, Nana, is contemplating "Papa-katsu" (sugar daddy arrangements) to earn money, a prospect that deeply distresses Ken'ya. He himself dropped out of high school to work multiple part-time jobs, but his earnings are meager.
His current income comes from exploring the second floor of the Dungeon, where he fights goblins. Despite his efforts, his earnings are insufficient. Ken'ya's profession is "Alchemist," a rare and initially considered useless job, as his primary ability is to refine and level up equipment. He possesses a status screen that displays his equipment and stats, which are currently low. He learns that progression in the Dungeon relies on acquiring better equipment and leveling up his profession's skills using Dungeon Points (DP).
A turning point occurs when Ken'ya accumulates 100 DP and uses it to level up his Alchemy skill. To his surprise, his Alchemy rank increases to D, allowing him to refine D-rank equipment. He discovers that his Alchemy skill can also "evolve" E-rank equipment into D-rank items. He successfully transforms an "Soldier's Gauntlet" into a "Knight's Gauntlet," a D-rank item worth a significant amount of money. This discovery ignites his hope for a better future and allows him to provide for his family.
Ken'ya's newfound ability allows him to earn enough money to buy groceries, including meat and eggs, a rare treat for him and Nana. He also confronts Nana about the "Papa-katsu" messages, reassuring her that he will earn enough money to support them. He then focuses on collecting and refining "Soldier" series equipment to create a full set of "Knight" series gear, significantly boosting his stats.
He visits the Dungeon's first floor, a hub for explorers, and interacts with his assigned navigator, Nakano Ai, a kind and supportive woman who takes a liking to him. Ai helps him acquire a "Steel Sword," a D-rank weapon, by covering the cost, with the understanding that he will repay her later.
Armed with his new equipment and sword, Ken'ya challenges the Dungeon's fifth floor, defeating the Hobgoblin boss with ease. He continues to gather "Soldier" series equipment, aiming to complete the "Knight" set and further improve his abilities. He also learns about the ranking system for explorers and the importance of equipment rarity.
The narrative then flashes back to Ken'ya's expulsion from the "Sugar & Salt" guild. His former guild leader, Satou Toshio, a wealthy and arrogant individual with a powerful "Holy Knight" job, mocks Ken'ya for his "Alchemist" profession and kicks him out, confiscating his equipment. This humiliation, coupled with the discovery of malicious rumors spread about Nana, fuels Ken'ya's anger and determination to become stronger.
Driven by this rage, Ken'ya dedicates himself to collecting and refining "Soldier" series equipment to create a complete set of "Knight" series gear. He successfully obtains all four pieces of the "Knight" set, significantly enhancing his combat capabilities. He then sets his sights on defeating the fifth-floor boss again to acquire a "Steel Sword" and further improve his offensive power. The chapters conclude with Ken'ya embarking on his quest, fueled by a desire for both financial stability and revenge.
