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Summer vacation is approaching, and elementary school student Keiichi Uehara finds a large stone on his way home from school. He brings the stone back home, and to his surprise, when he washes it with water, a small kappa (river imp) jumps out. The kappa "gugs" at Keiichi, so he names it "Kogu." Kogu can speak human language and has been buried underground for hundreds of years. Keiichi quickly becomes friends with Kogu, and they bathe, eat, and sleep together. The rest of the family also accepts Kogu, and the kappa becomes a secret of the Uehara family. One day, Kogu expresses a desire to return home to be with its companions. To let Kogu understand the outside world, Keiichi secretly takes it out. Kogu is shocked to find the environment completely changed, and it can't find any companions in a place unsuitable for kappas to live. Seeing Kogu's distress, Keiichi feels bad as well. He and Kogu embark on a journey together, arriving at Iwate Prefecture, where the legend of kappas is still preserved. They find a suitable place for kappas to live in the beautiful natural environment, and Kogu happily swims in the clear river, making Keiichi happy for it. However, they still cannot find any other kappas there. Keiichi overhears villagers saying, "If you catch a kappa, you can earn 10 million yen!" and quickly takes Kogu away from there. Upon returning home, Keiichi is surprised to find a young reporter. It turns out that the reporter heard about the Ueharas keeping a kappa and insists on taking photos. Kogu's secret is finally revealed to everyone. Keiichi's father is ordered by his company to have the kappa appear on television. To protect the Uehara family, Kogu agrees to participate in the TV program. However, among the guests present, it sees someone who looks very much like its father's murderer. Just as Kogu is surrounded by fear, an accident occurs at the TV program site. It desperately escapes to the outside, but wherever it goes, it sees crowds of people. To escape the humans who terrify it, Kogu climbs the Tokyo Tower. The summer sun exhausts it. Kogu looks down at the city and whispers to itself, "There's no quiet place to be found here. This is the nest of humans. I can't go on. I'm going to my father..."