Kouichi Uehara is a fourth grade student living in the suburb of Tokyo. One day, he picks up a large stone which turns out to be a fossil of a baby "Kappa" who has been sleeping underground for the past 300 years, and names it "Coo." They become good friends and Coo starts living with Kouichi's family. However, Coo has a hard time adjusting to life among humans and misses his kind.

The film won the Best Animation Film award at the 2008 Mainichi Film Awards and was nominated for Best Animation Film at the 2008 Japanese Academy Awards, and for Best Animated Feature Film at the 2007 Asia Pacific Screen Awards. It also won the Grand Prize for Animation at the 11th Japan Media Arts Festival and the Feature Film category at the 2008 Tokyo Anime Award.