Akito Fujimura, a thirty-two-year-old man who is self‑acknowledged and widely recognized as a corporate slave, finally returns home after days of sleepless forced labor. Exhausted, he finds a bizarre structure that looks like a dungeon sprouting in his garden. Suspecting it might be a television stunt, he nonetheless enters and soon encounters a goblin. Even after days without rest, he remains unflappable, and in the struggle he manages to defeat the monster. That day marks a turning point: the world abruptly transforms into something resembling a game. Dungeons appear across cities, monsters emerge and roam freely, and humanity is thrust into a new reality where leveling up and racial evolution become possible.

Amid the chaos, Akito continues working his grueling office job while quietly exploring the dungeon that appeared in his own yard. He treats his dungeon excursions as a form of training, steadily growing stronger. Through persistent effort and a calm, methodical approach, he achieves racial evolution—a metamorphosis that grants him enhanced abilities and pushes him toward the pinnacle of power. His unflappable nature and sheer endurance, forged by years of overwork, become unexpected assets.

As Akito delves deeper into the endless corridors of the dungeon he calls the Infinite Corridor, a global event known as the Chaos Welt beta test triggers a final quest: save Chrono Princess, a mysterious black cat. Akito successfully rescues the cat, and upon clearing the quest the world shifts into its next phase. All of humanity now gains the capacity for racial evolution, upending daily life even further. Despite the upheaval, Akito continues to train and explore, steadily climbing the stairs toward ultimate strength.

Later, Akito re‑encounters a former high‑school classmate, Natsudouin Yayoi, now a teacher at Seika Girls High School. She has discovered that Akito is secretly the caped hero known as Hoppy, who has been making headlines. She asks him to investigate a student disappearance at her school. Akito agrees, and his search leads him into a conflict involving the oni realm. The demon king of the oni, Gaki‑ō, had orchestrated a human invasion game called Chaos Welt. After humans defeated his forces, the demon prince Yasha‑dōji is punished by having his parents killed. To protect his younger sister Saya, Yasha‑dōji seeks a human who can serve as a gate between worlds. He finds Azusa Amamiya, Akito’s colleague, who bears a striking resemblance to Saya. The oni capture Azusa and use her to open a gate, unleashing a horde of demons into the human world. The final confrontation becomes a war game between humans and oni, with Akito at the center of the battle to save humanity.

Throughout these arcs, Akito remains the same unshakable corporate employee, dividing his time between office work, dungeon training, and gradually becoming the strongest being on the planet. The story is presented as a heroic tale spun by a group of dangerous eccentrics—a worst and worst kind of hero saga where a company slave evolves his race and ascends to the peak of power.