Dungeons: mysterious labyrinths connecting our world to another dimension. Lured by the promise of limitless resources, humanity eagerly explored them—unwittingly triggering a monstrous invasion that pushed the world to the brink of extinction.

To save humanity, the world’s strongest "Buster," Kiren Tsurugi, faced the cataclysm alone... only to vanish without a trace.

Years later, a new wave of monsters attacks. But standing in their path is a single, mysterious young boy—a boy who bears a striking resemblance to the legendary Kiren.