After heroically sacrificing himself to save a girl from a monster, high schooler Kazuki Muto awakens in disbelief, thinking it was all a dream until a real-life threat emerges. Saved by the mysterious Tokiko Tsumura, he learns of his revival through an alchemical device called a Kakugane, which grants him a powerful weapon known as a Buso Renkin. With Tokiko by his side wielding her own Buso Renkin, they unite to battle sinister homunculi and their malevolent creator, embarking on a perilous journey to protect humanity.