Kôichi Uehara is a fourth grade student living in Higashikurume City, a suburb of Tokyo. One day he picks up what he thinks to be a fossil of an ancient creature but turns out to be a baby kappa (a mythical Japanese water creature) that was sleeping underground for the past 300 years, and he names it "Coo". Kôichi and Coo become good friends and the kappa starts living with the human boy's family. However, Coo cannot not fully adjust to life in present-day Higashikurume and starts to miss his own family. On a hot summer day, Kôichi and Coo decide to go on an adventurous road trip to find his own kind.