In this world, there is another world besides that of humans; the world of beasts.
In the city of Jutengai in the world of beasts, the Lord, who has brought the beasts together under his leadership for a long time, has just declared his retirement and reincarnation into a god. The tradition is that the successor should be someone who excels in strength and dignity, and it is determined that the next Lord would be chosen at a match to be held in the arena in a few years' time. The candidates are Kumatetsu who is simply strong but crude, and Iozen who has both strength and dignity. As the race for the next Lord starts to be in full swing, Kumatetsu is given a task by the Lord to take an apprentice.
Meanwhile, in Shibuya, Tokyo in the human world, a 9-year-old boy Ren is spending his days all alone, having been separated from his father due to his parents’ divorce, and also bereaved of his mother.
One night, while aimlessly wandering the streets, Ren encounters Kumatetsu, who has come to look for an apprentice, and winds up in Jutengai. Wanting more than anything to have the strength to live on his own, Ren decides to become Kumatetsu’s apprentice, and is given a new name, Kyuta (‘kyu’ means the number ‘nine’ in Japanese).
At first, Kumatetsu and Kyuta butt heads on just about everything, but they grow together as they go through their odd life of co-living and training. As the days go by, a bond like a real father and son starts to form between them.
On another front, Iozen also had a son, Ichirohiko, who was around the same age as Kyuta. Ichirohiko is proud of his father more than anything else, and wishes to be like his father. However, he has been troubled by the fact that his beast-like tusks have still not grown.
Time passes, and Kyuta and Ichirohiko both grow into young adulthood. The 17-year-old Kyuta is now Kumatetsu’s number one apprentice and known for his strength, but he has been questioning his existence, wavering between his beast and human identities.
One day, he accidently wanders back to Shibuya, where he meets a schoolgirl named Kaede. Through Kaede, Kyuta learns about a new world and seeks for his way of life.
Finally, it is the day of the match to decide the next Lord – and a crisis involving both beasts and humans befalls the two worlds.
What is it that I can do in order to save everyone? The time has come for both Kumatetsu and Kyuta to each make their decision.