In Buso Renkin, Kazuki Muto, a high school student, meets his demise while attempting to rescue a girl from a monstrous creature. The next day, he questions the reality of the events, only to find out they were all too real. The girl, Tokiko Tsumura, reveals herself as a warrior on a mission to exterminate creatures known as homunculi, who feed on humans. To bring Kazuki back to life, Tokiko implants a mystical device called a 'kakugane' in his chest, granting him the ability to materialize a lance for combat. As Kazuki delves deeper, he learns of Tokiko's affiliation with the Alchemist Soldiers, a group dedicated to defending the world against these malevolent entities. United in purpose, Kazuki and Tokiko embark on a perilous journey to confront the dark force behind the creation and manipulation of the homunculi.