Kouichi Uehara, a fourth‑grade student living in the suburbs of Tokyo, discovers a large stone that turns out to be a fossilized baby Kappa that had slept underground for 300 years. He names the creature Coo. As Koichi and Coo become close friends, Coo joins the Uehara family, but struggles to adjust to life in the city and longs for his own Kappa kin. One hot summer day, the two decide to embark on an adventurous road trip to find Coo’s fellow kappas.