A world where the machine civilization has perished and immortal monsters called "Monotsuki" ("Possessed") are rampant. Isana, a girl born in a remote village, meets an android who lost her memories and looks exactly like her sister, who should have disappeared in the ruins she wandered into. Convinced that her sister is alive, Isana gives the mechanical person the name "Hime" ("Princess") and they set out together on a journey to find her sister.