This anime film, written and directed by Mamoru Hosoda, blends together epic fantasy and a coming-of-age tale. A nine-year-old boy named Kyuta finds a mysterious passageway to a world inhabited by beasts, where he is chosen to serve as the apprentice to a legendary warrior called Kumatetsu. Eight years later, Kyuta is torn between helping Kumatetsu battle a rival for control of the beasts' realm, or returning home to Tokyo to resume his old life there.