In a tale that threads centuries of myth into a modern setting, a kappa from the past resurfaces as a quiet, life-changing presence for a Tokyo family. After a dreadful encounter with a samurai, a young kappa's father is murdered; as he cries, the ground opens up and swallows him. Fast forward to today, where Koichi Uehara Takahiro Yokokawa and his friends pressure him to tease the quiet Sayoko Kikuchi, whom they deem ugly. A dare lands Koichi in a muddy ditch where he discovers a stone containing the kappa. He brings the creature home, and the household is awed by their unusual guest. The kappa, whom they name Coo, forms a quick bond with the family, though Koichi's younger sister remains wary at first. Their loyal dog Ossan, capable of telepathic communication with Coo, keeps that bond a secret.
From the moment Coo appears, the Uehara home becomes a nexus for strange happenings. A sighting by a couple draws media attention and rumors of a kappa spread across Japan. Curious about his origins, Koichi decides to investigate the kappa’s past with Coo, traveling to the countryside where Coo talks with a Zashiki-warashi, a house spirit child. She suggests that Coo may be the last of his kind, a clue that deepens his sense of belonging and responsibility. When they return, photographers and journalists camp outside the house, forcing the family to confront fame. To placate the press, Yasuo Uehara Naoki Tanaka records a video of Coo to pitch to potential buyers and viewers. Through Ossan’s knowledge, it’s revealed that Coo had an earlier owner who loved him but grew to treat him harshly under the stress of school life; eventually the kappa ran away and was adopted by the Ueharas.
Coo agrees to appear on television alongside the Ueharas, bringing Ossan along as a companion. On the talk show, a cryptid expert—who bears a striking resemblance to the samurai—suggests that the samurai took Coo’s arm after striking him down. Coo recognizes the arm as his father’s and breaks down in tears, using his powers to plunge the studio into darkness and chaos. He escapes with Ossan atop the collapsing set, carrying the arm as bystanders pursue them. The chase takes a tragic turn when Ossan is struck by a car and dies. Coo indulges a violent impulse to destroy a bird planning to prey on Ossan, then climbs a radio tower with his father’s arm in tow. As he contemplates his fate, a sudden rainstorm gives way to a water-spirit dragon that persuades him to keep living, and he is rescued.
Meanwhile, Koichi’s evolving kindness extends to Sayoko Kikuchi, and he defends her honor when tensions rise. Sayoko later reveals she must move away because her parents are divorcing. Back at home, Yukari informs Koichi that Coo will be leaving, having received a Yokai-signing letter. The Uehara family grieves but chooses to honor their bond with Coo, spending one last day together. Yasuo creates a distraction that allows Koichi to ship Coo off, and the two share a final moment at the doorway before parting ways. The promise to write lingers between them as Koichi undertakes a long journey to Okinawa to deliver Coo to his new caretaker.
In Okinawa, Coo’s recipient is revealed to be a kijimuna, a kappa-related cousin who saw the news and grew worried. He pledges to teach Coo how to alter his appearance and affectionately tells him he can call Ossan. Venturing into the lush landscape, Coo—still carrying his father’s arm—begins a new chapter, determined to seek out more kappas and, someday, to reunite with the Ueharas.
Throughout, the narrative blends themes of wonder and loss, highlighting a boy’s empathy, a family’s resilience, and a creature’s coming‑of‑age journey across a world where myth quietly walks beside the everyday.
