The story involves a nine-year-old boy named Ren, who, after the untimely death of his mother after a bitter divorce, ends up filled with despair and rage, roaming the streets of Tokyo rather than live with his distant relatives.  That's when Ren is spotted by a Kumatetsu, a hot-tempered ursine beast from another dimension where animal-men rule supreme, Jutengai.  Ren ends up following Kumatetsu into an alley that serves as a portal back to the a supernatural place where shape-shifting humanoid animals with otherworldly powers, and where humans are not allowed because of their dark predilection for death and destruction.  An exception is made for Ren, redubbed as Kyuta (for his age, "nine") who has been taken in, rather reluctantly, as the Kumatetsu's apprentice, and given the undisciplined beast-man is one of two possible successors that will ascend to the rule of the fantastical domain, he has some leeway in this regard.  Both are ill-equipped to handle their newfound roles as mentor and apprentice, especially as they butt heads constantly, but they soon learn to make it work to their benefit and actually do some learning and growing from one another.