The story begins with the protagonist, Hotaru Amekawa, a self-proclaimed NEET and "home security guard," suddenly plummeting into a dark abyss after a mundane morning run. He lands safely in a simple earthen room, where a holographic projection of King Louis from another world appears. Louis explains that their world is under siege by powerful magic beasts and dungeons, and as a desperate measure, they've connected dungeon entrances to other worlds, including Hotaru's. Hotaru is tasked with clearing the dungeon and is granted skills and magic by the gods.
Initially confused, Hotaru quickly discovers his "Item Box" and "Status" screen, revealing game-like abilities: "Otherworld Appraisal," "Item Box Lv.max," and "Water Magic Lv.4." He ventures into the dungeon, encountering goblins. His first combat is a mix of fear and strategic thinking, using his "Water Jet" magic to trip enemies and then weapons from his Item Box to defeat them. He learns that his actions increase his strength and speed. After a challenging boss fight against a Goblin Captain, he receives a "Select Skill Scroll," choosing "Super Dynamic Vision," and also passively gains "Early Bloomer," significantly boosting his growth and combat prowess. His Water Magic also expands, granting him new offensive and binding spells.
As Hotaru delves deeper, progressing through floors 2 to 50, his new skills make combat against various goblins and White Dogs almost effortless, leading him to describe it as a "null-game." He discovers "rest areas" every five floors, providing amenities like a kitchen and bed, allowing him to cook and eat the dropped goblin meat. On floor 16, he encounters Kuu, a "Cold Fox" spirit, which he befriends by offering a rare gem. Kuu forms a contract, becoming a temperature-regulating scarf and sharing its "Ice/Snow Magic" through a "Coexistence and Co-prosperity" skill, further diversifying Hotaru's abilities.
However, his progress halts on floor 51, where he encounters "Beetle Larvae" resistant to both water magic and physical attacks. Forced to retreat, Hotaru decides to grind on the previous floor's boss, the "Greedy Nouveau Riche Pig," to level up.
Meanwhile, a parallel narrative unfolds on Earth. Goblins suddenly appear globally, causing panic. Conventional weapons prove ineffective. High school student Kotaro Awaya, trapped in a school bathroom, resourcefully defeats a goblin using a keychain and the monster's own spear. He's brought before government officials, including Nagase Jiro. Kotaro reveals the "Status Open" command, demonstrating the game-like system, and theorizes that dungeons are the source of the monsters. He proposes a new strategy: using monster weapons and close-quarters combat. The government adopts this, successfully containing the goblin threat in most of Japan, leading to the rise of "Dungeon Adventurers."
Back in the dungeon, Hotaru's grinding pays off. His skills and magic level up dramatically. On floor 75, he defeats a boss, the "Red Crows," and obtains "Guaranteed Random Skill Scrolls," gaining "Shape Manipulation" (for magic) and "Magic Power" (boosting MP and spell strength). He then finds a "Spirit Sealing Stone," releasing Pon, a "Thunder Raccoon" spirit. Pon forms a contract, becoming sporty clothing and sharing "Electric Magic," including the powerful "Electric Speed" spell.
With three magic types, Hotaru shifts his combat style, becoming a versatile magic-melee fighter. He continues his descent, easily overcoming new monster types and even acquiring "Heavenly Feet" equipment for aerial movement. On floor 155, he faces a flying "Poison Snake Bat," which he defeats using a combination of his new aerial mobility, Ice/Snow magic, and a devastating Electric magic combo. He gains "Super Balance Sense," further enhancing his close combat and acrobatic abilities, solidifying his transformation into a formidable adventurer. The dungeon's end remains elusive, but Hotaru, now accompanied by his spirit companions, is more powerful and adaptable than ever, ready for the challenges ahead.
