Nine-year-old Ren is grappling with the tragic loss of his mother, having no word from his father and resolutely rejecting the idea of living with his legal guardians. In his sorrow, he escapes onto the bustling streets of Shibuya, where he resorts to stealing food and sleeping in alleys, haunted by memories of his mother's funeral.
Meanwhile, in the mystical Beast Kingdom, the grandmaster has announced his retirement with plans to reincarnate as a deity. He identifies two potential successors: the well-liked Iōzen, who juggles fatherhood with his duties, and the formidable yet slothful Kumatetsu, whose solitude weighs heavily on him. The grandmaster encourages Kumatetsu to find a disciple in hopes of igniting a spark within him to succeed.
As Kumatetsu wanders Tokyo alongside his companion, Tatara, he crosses paths with Ren and proposes that the boy becomes his disciple. Initially resistant, Ren is drawn in by curiosity and reluctantly ventures with Kumatetsu back to the Beast Kingdom, where he finds himself unable to return to the human realm. He witnesses a fierce battle between Iōzen and Kumatetsu, marveling at Kumatetsu's unwavering spirit despite the lack of support. However, Kumatetsu suffers a swift defeat, though the grandmaster reassures them that the true duel for succession has yet to unfold.
Kumatetsu names Ren "Kyūta," respecting his decision to withhold his real name. Their early training sessions are fraught with difficulties, but Kyūta soon realizes he can learn by mimicking Kumatetsu during daily chores. As he becomes adept at foreseeing his master's movements, their relationship deepens, and they embark on rigorous training together. Eight years later, the teenage Kyūta emerges as a skilled kendo practitioner, while Kumatetsu finds new purpose and a group of supporters, including Jirōmaru, who aspires to train under him.
Eventually, Kyūta finds a way back to the human world and befriends Kaede, a fellow student. Amidst reconnecting with his father—who has been on the lookout for him since his disappearance—Kyūta struggles to balance his dual existence and confronts the bitterness he feels as Ren, alongside the disconnection he experiences as Kyūta. When he alienates both his father and Kumatetsu, he discovers an overwhelming emptiness within, until Kaede helps him regain composure, presenting him with a bracelet that has brought her comfort.
On the day of the crucial succession duel, Kumatetsu falters without Kyūta's encouragement and is nearly overpowered by Iōzen. However, Kyūta secretly observes the fight and ultimately reveals himself, bolstering Kumatetsu's strength and aiding him in vanquishing Iōzen. Following Kumatetsu's victory, it is unveiled that Ichirōhiko, long thought to be a mere beast, was raised by Iōzen after being discovered as an infant on the streets of Tokyo. Like Kyūta, Ichirōhiko grapples with an internal void, resulting in his unwillingness to accept his humanity, ultimately manifesting terrifying telekinetic powers that inflict grave harm on Kumatetsu.
As Kyūta wrestles with the temptation to unleash his own emptiness in an attempt to annihilate Ichirōhiko, he finds clarity through Kaede's bracelet. Meanwhile, Kumatetsu embraces his role as the new grandmaster, reincarnating as a sword "to be handled with the heart," reminiscent of their initial training sessions. By merging with Kyūta, he fills the void inside him, and together they combat Ichirōhiko’s darkness. In the aftermath, Ichirōhiko awakens surrounded by his adoptive family, blissfully unaware of the turmoil that transpired.
Back in Shibuya, Ren engages in a heartfelt conversation with Kumatetsu, whose spirit now resides within him, exchanging promises of support and gently threatening him to remain strong. Following this powerful experience, Ren observes a celebration of victory with Kaede in the Beast Kingdom. In reconciling with both his father and his own identity, he resolves to return to the human world, carrying Kumatetsu's essence in his heart forevermore.
