Kouichi Uehara, a fourth grade student in elementary school, lives in a suburb of Tokyo. One day, walking back from school along the river, Kouichi kicks up a large stone which looks like a fossil. It turns out to be a baby “Kappa”– a Japanese mythical water creature called Coo who was sleeping underground for the past 300 years. Kouichi and Coo become good friends and live together with Kouichi’s family. However, Coo cannot adjust to the lifestyle in Tokyo and so, on a hot summer day, Kouichi and Coo decide to go on an adventurous road trip to find Coo’s peers.