In a fantasy world where a divine system assigns a fixed role to every person at birth, most are given the title of Villager, the weakest class with no combat abilities, leaving them to live quiet lives of farming and trade. The powerful roles of Warrior, Fighter, Cleric, Magician, Rogue, Merchant, Hunter, and Sorcerer are reserved for a few, while the legendary classes of Royalty, Hero, and Sage are even rarer. For a Villager, hunting monsters is equivalent to suicide, and the average person never exceeds level five. However, one young man named Koji Kagami, born a simple Villager, discovers an exception to the rule. After realizing that defeating monsters yields great wealth, he dedicates his life to fighting. Through relentless effort and by exploiting a system that grants more experience when fighting at a disadvantage, Kagami reaches the absolute limit of level 999, becoming the strongest being in a world where even legendary heroes rarely surpass level 450.

Having achieved this pinnacle, Kagami grows bored and disillusioned, finding no further purpose in his strength. His life changes during a chance encounter in a cave with a young demon girl named Alice Barunesio. As the daughter of the Demon Lord Douglas, Alice is the natural enemy of humanity, yet she has been taught by her father not to hate humans and instead dreams of achieving coexistence between the warring races. At that moment, the Hero Rex Nipplesboy, driven by a blind hatred after his parents were killed by demons, is hunting Alice with his party, which includes the brainwashed Sage Kururu Hexaldoria and the priestess Tina Bills. Kagami effortlessly intercepts the heroes, not as an enemy, but as an undeniable force that forces everyone to reconsider their prejudices.

Recognizing a shared sense of isolation from their respective societies, Kagami joins forces with Alice and her loyal demon servant, Menow Abrasha. Together, they embark on a journey to challenge the world's rigid fate system, which insists on eternal conflict between humans and demons. Their party expands to include unconventional allies such as the comically muscular female fighter Takako Builder, who is convinced of her own attractiveness, and eventually the heroes themselves, who come to understand Kagami's worldview. The narrative goes beyond simple monster battles to expose a deeper corruption. The King of Hexaldoria is revealed to be a "Singularity," an entity maintaining the world's function, and clues suggest the entire reality might be an artificial construct or a game, with past civilizations of Japan, the USA, and Russia having perished to create the current magical world. As the story progresses toward the 2026 anime adaptation, Kagami and Alice must fight not only the monsters and the hero party but also the transcendent beings and the system itself to create a future where a Villager and a Demon Lord's daughter can coexist. The series, which concluded its light novel run across eight volumes, also spawned a long-running manga adaptation that won the grand prize at the 2019 Electronic Manga Awards.