Summer vacation is coming soon. On his way home from school, Koichi Uehara, an elementary school student, picked up a big stone. He took the stone home, but when he washed it with water, a little kappa jumped out of it. The little kappa called him "Gu", so Koichi named it "Xiao Gu". Xiao Gu can speak human language, and he has been buried underground for hundreds of years. Koichi soon became good friends with Xiao Gu. They took a bath together, ate together, and slept together. Other family members also accepted Xiao Gu, and Xiao Kappa became a secret of the Uehara family. One day, Xiao Gu proposed to go home and be with his friends. In order to let Xiao Gu understand the outside world, Koichi secretly took it out. Xiao Gu was shocked to find that the environment had changed beyond recognition. This place that was not suitable for the survival of kappa could not find any partners at all. Seeing Xiao Gu sad, Koichi was not happy either. He went on a trip with Xiao Gu and came to Tono Prefecture, where the legend of Kappa was still preserved. The two found a place suitable for kappa to live in the beautiful nature. Xiaogu happily swam in the clear river, and Koichi was happy for it. However, there were no other kappas here. Koichi heard the villagers say "Catching a kappa can earn 10 million!", and hurriedly took Xiaogu away. When he returned home, Koichi was surprised to run into a tabloid reporter. It turned out that the man heard that the Uehara family had a kappa and had to take a photo. Xiaogu's secret was finally known to everyone. Koichi's father was ordered by the company to make the kappa appear on TV. For the sake of the Uehara family, Xiaogu agreed to participate in the TV program. However, among the guests present, he saw a person who looked like the murderer of his father. Just when Xiaogu was surrounded by fear, an accident happened on the TV show. He desperately fled outside, but no matter where he ran, he would see a dark crowd. In order to get rid of the humans who scared him, Xiaogu climbed up the Tokyo Tower. The scorching sun in summer exhausted him. Little Gu looked down at the city and whispered to himself, "There is no quiet place anywhere. This is the nest of humans. I can't do it anymore. I want to go to my father..."