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 person who lost her memories and looks exactly like her sister, who should have disappeared in the ruins she wandered into. Convinced that "My sister is alive," Isana gives the mechanical person the name "Hime" (princess) and together they set out on a journey to find her sister.