Après le décès de sa mère, Ren, 9 ans, refuse de vivre avec sa famille éloignée et prend la fuite. À la nuit tombée, alors qu'il arpente seul les rues du quartier de Shibuya, il est interpellé par un étrange personnage encapuchonné... Sous la cape se cache Kumatetsu, une bête au corps imposant et aux dents aiguisées venue d'un univers parallèle au nôtre. Transporté dans ce monde fantastique peuplé de bêtes étranges, le jeune garçon va se retrouver à son insu disciple de cet ours tumultueux ! Il devra désormais apprendre à composer avec son nouveau maître au tempérament... bestial !

Having accidentally wandered into the beast world of Jutengai, Kyuta has gotten himself a master—Kumatetsu. But it turns out Kumatetsu's not quite as good at teaching as he is at brawling. The Grandmaster sends teacher and student alike on a pilgrimage, asking all the wisest beasts in the world a simple What is true strength? But are the boy and the beast ready for the answer?

Kyuta has been spending more and more time back in the human world. He's about to take an exam that will let him go to college! But does this mean saying goodbye to his master and his life in Jutengai forever? Can he be a part of both worlds, or will he have to choose...?

Ichirohiko has suffered, caught in the space between human and beast, and has succumbed to the darkness. Kyuta leaves Kumatetsu, on the brink of death, in the care of the others and heads to the human world in pursuit of Ichirohiko. It's the start of a life and death battle in the streets of Shibuya to settle things not only with Ichirohiko, but with the darkness in his own heart...!! What can he do to save everyone? The times has come for Kumatetsu, Kyuta, and Kaede to each make their own decisions...