On his way home from work, salaryman Kazuto Kudou runs over a large, mysterious dog with his car. Instantly, a disembodied voice echoes in his mind, announcing monster defeat and experience point gain. When he wakes the next morning, the world has transformed overnight into a landscape overrun with vicious monsters, now governed by a pervasive game-like system of levels, skills, and statuses. Facing this dangerous new reality, Kazuto must rely on a special skill he acquired the previous night to survive, level up, and navigate a modern society turned into a fantasy deathscape.

The setting is a contemporary Japanese city that has abruptly merged with fantasy rules, where ordinary streets and buildings become hunting grounds. Kazuto Kudou is the main protagonist, an ordinary office worker forced to rapidly adapt to the lethal logic of experience points and progression. The central conflict revolves around his struggle for survival against overwhelming monster attacks while also contending with the collapse of normal society. A key narrative element is the unidentified unique skill he possesses, which is implied to be a crucial advantage for advancement in this brutal environment.

Notable narrative arcs follow Kazuto as he learns to navigate and exploit the new system, moving from panicked flight to strategic combat. The story explores the transformation of a realistic urban landscape into a level-grinding arena. As he gains levels and confronts increasingly powerful named monsters, the manga highlights the isolating struggle of an individual leveraging game mechanics to live according to his own wishes in a world that has completely abandoned its former rules. The series focuses on the core loop of defeating monsters, gaining experience, and unlocking new abilities to face escalating threats.