In a quiet Tokyo suburb, young Koichi Uehara discovers an ancient kappa, a mythical water creature named Coo, while playing near a riverbank. Coo had been buried underground for centuries and is grateful to Koichi for setting him free. Taking Coo home, Koichi introduces the kappa to modern life, and despite initial hiccups, Coo gradually adapts to his new surroundings. The family grows fond of Coo, treating him almost like another member of the household. Through his interactions with Coo, Koichi learns about the kappa's lost world and the legends surrounding his species, which have all but disappeared in the modern era.

As their bond deepens, Coo’s presence attracts significant attention, and not all of it is friendly. The tranquil summer days give way to challenges as the media and curious onlookers become aware of Coo's existence, leading to complications that strain the Uehara family’s peaceful life. While Coo tries to navigate his newfound friendships and adapt to this strange era, Koichi finds himself torn between protecting his mythical friend and dealing with the ever-growing disruptions from the outside world. Together, they must figure out how to handle the shifting dynamics and preserve the sanctity of their companionship amidst increasing external pressures.