The protagonist, Koji Furuya, a second-year university student, faces financial ruin after losing his entire one million yen savings in his first FX trade. His dream of early retirement through FX is shattered. Despondent, he contemplates his next move, considering returning to part-time work or even starting a video channel about his misfortune. Suddenly, he gains a skill called "Magic-Based Arbitrage within Dungeons."
Investigating this new ability, Koji discovers that acquiring a skill or entering a dungeon allows one to view their "Status." He activates this, revealing his stats and the new skill, along with a basic active skill, "Mana Ball." Upon examining "Magic-Based Arbitrage within Dungeons," he learns it allows him to absorb excess magic from low-density areas in dungeons and transfer it to high-density areas, converting the difference into his own magic power. Crucially, his MP becomes infinite while inside a dungeon. This discovery ignites his determination to explore dungeons.
The next day, Koji visits a dungeon entrance. He learns that first-time explorers must be accompanied by a "Guide" who assesses their capabilities. He meets Kudo, his guide, a burly man who expresses concern about Koji's pale complexion, which is due to his MP being depleted from practicing "Mana Ball" before entering. Upon entering the dungeon, Koji's MP instantly replenishes to infinite, as predicted.
Kudo demonstrates how to defeat low-level monsters, like the "Rat-Mouse," using basic weapons and explains the concept of "drop items" like magic stones. Koji then uses his "Mana Ball" to defeat a Rat-Mouse, impressing Kudo with his ability to do so with minimal MP expenditure and recover it instantly. Kudo, recognizing Koji as a "skill user," allows him to fight a higher-level monster, a Lesser Rock Golem, to assess his potential. Koji, limited by the maximum power of a single "Mana Ball" to his current MP capacity, resorts to rapid-fire attacks, dubbed "gummy-shooting." This barrage overwhelms the Golem, causing Koji to level up and evolve "Mana Ball" to "Mana Ball V2." Kudo is astonished by Koji's rapid progress and unique abilities.
Koji is granted a permit to explore up to the 7th floor. He then proceeds to hunt Lesser Rock Golems on the 4th floor, efficiently farming magic stones and a leather armor drop. His "Mana Ball V2" evolves to "Mana Ball V3" after a significant number of uses, gaining the ability to penetrate magic immunity. He also acquires "Dowsing Wires," which enhance monster detection, and subsequently evolves it into the "Total Floor Detection" skill, granting him a map of the entire floor and enemy locations.
Koji continues his rapid progression, reaching level 12 and acquiring the "Gatling Knuckle" gloves, which allow him to fire his "Mana Ball V3" at an incredibly high rate without MP depletion, effectively multiplying its power. He sells his accumulated magic stones, earning a substantial amount, and his permit is further extended to the 10th floor due to his exceptional progress and the new skill. He decides to explore the 5th floor, which contains wolf-like monsters, and plans to continue his aggressive farming strategy.
