A world where machine civilization has perished and immortal monsters called "Monotsuki" are rampant. Isana, a girl born in a remote village, meets a mechanical man who looks exactly like her sister, who should have disappeared in the ruins she wandered into, and has no memory of her. Convinced that "her sister is alive," Isana gives the mechanical man the name "Hime" and together they set out on a journey to find her sister.