There is a world of “beasts” that coexists with the world of humans. In the city of Shibuya, there is a parallel world called “Jutengai,” a society of 100,000 beasts where its leader, the Lord, who has just declared his retirement and reincarnation into a god, begins looking for his successor. He has two candidates—Iozen, an honorable and popular beast, and Kumatetsu, strong but crude and without a single apprentice.

Ren, a 9-year-old boy, whose father left his family and whose mother recently died in a traffic accident, is all alone. Refusing to be taken in by relatives, he aimlessly wanders the streets of Shibuya and encounters Kumatetsu, who has come to the real-life world to look for an apprentice. Ren, who wants more than anything to be strong, follows Kumatetsu to Jutengai. There, he becomes Kumatetsu’s apprentice and is given a new name, Kyuta. At first, Kumatetsu and Kyuta butt heads on just about everything, but gradually they settle into an odd life of training alongside companions Tatara and Hyakushubo. As the days go by, Kumatetsu and Kyuta—both lonely souls—grow together, and a bond like a real father and son starts to form between them.

One day, when Kyuta has grown to be a strong, young man, he accidently wanders out of Jutengai back to Shibuya, where he meets a schoolgirl named Kaede. Through Kaede, Kyuta learns about a world and values he knows nothing about and begins asking himself in which world he really belongs.

Meanwhile, Kumatetsu and Iozen fight a battle over the lordship. Empowered by Kyuta’s support, Kumatetsu defeats Iozen, but Ichirohiko, Iozen’s son whose heart has been possessed by darkness, refuses to accept his father’s defeat and retaliates. Chaos befalls the two worlds of Jutengai and Shibuya. What can be done to save everyone? Kyuta bids farewell to Tatara and Hyakushubo and heads to Shibuya for a face-off with Ichirohiko.

Kumatetsu, Kyuta, and Kaede. The time has come for each of them to make a decision.