In the fantasy world of Earthqualia, every person is assigned a role at birth that determines their path in life. The powerful are given classes like Warrior, Sage, Hero, or Royalty, while the vast majority of the population receives the weakest designation of all: Villager. For a villager, who has no special skills or combat abilities, leaving the safety of town to hunt monsters is essentially a death sentence. Most people never exceed level 5.

The protagonist, Koji Kagami, was born with the villager class, yet he has somehow reached the maximum level of 999, making him an anomaly that shatters the world's rigid system. Through relentless and dangerous fighting against overwhelming odds, he has gained power that far surpasses even the legendary heroes of history, all while holding the lowest possible social rank.

Koji's life of tedium changes when he encounters Alice, the daughter of the Demon Lord. Contrary to the hatred that humans and demons hold for one another, Alice desires coexistence between the two races. Recognizing a kindred spirit who also defies the world's expectations, Koji decides to help her. Together, they embark on a difficult journey to bridge the divide between humans and demons, challenging the prejudices and political systems that perpetuate an endless cycle of conflict.

Other key characters include Rex, the Hero who was born with the divine mission to defeat the Demon Lord and has a deep-seated hatred for demons, and Kururu, the Sage who accompanies him. They initially view demons as pure evil but eventually come to understand Koji's perspective. Takako Builder is a female fighter of enormous size and strength who joins their cause.

The narrative spans several major arcs. After Koji meets Alice, they travel to the city of Varman to raise funds by opening a casino, where they encounter a mysterious and sinister butler named David. Meanwhile, in the royal capital of Hexaldoria, Koji's ally Paruna struggles with her own loyalties and doubts. The party eventually confronts the Dark Dragon, one of the most powerful monsters in the world, and begins to unravel the mystery of why their reality functions like a video game. This investigation leads them into direct conflict with the human king, revealing the dark truths of how the world operates and forcing Koji to carve a path against a fate that has been predetermined for him and everyone else.