Set in an alternate version of the Meiji Era, a boy named Ry?nosuke Sakamoto returns to Kyoto from Shanghai with his monster (y?kai) Nekomaru. The law states that in order to live freely with humans, a y?kai must be licensed. The Kid? Shinsengumi Co. Ltd. is responsible for the capture of unlicensed y?kai but must fight with the Tsubamegumi, who resent the new law. It turns out that Ry?nosuke is the son of the owner of the Shinsengumi.