Nine-year-old Ren loses his mother and bolts into the crowds of Tokyo's Shibuya rather than live with relatives he despises. There he stumbles into a hidden beast kingdom and becomes the reluctant disciple of Kumatetsu, a rough, short-tempered warrior who renames him Kyuuta and starts training him to fight. Years into his apprenticeship, Kyuuta slips back into the human world and befriends a girl named Kaede, caught between the two lives now shaping him. This manga adapts the 2015 animated film of the same name.