Koichi Uehara, a fourth-grader in Tokyo’s Higashikurume City, begins as an impressionable child swayed by peers but evolves after unearthing a fossilized kappa in a muddy ditch. Naming the creature Coo, he revives it by washing away centuries of sediment, sparking a journey fueled by growing empathy and resolve.

His family’s pragmatic support shapes his actions: a father nonchalantly tolerates Coo’s presence, a mother balances wariness with care, and a younger sister shifts from suspicion to affection, mirroring their collective adaptation to the mythical guest. Koichi’s instinct to shelter the vulnerable extends beyond Coo, evident in his rescue of Ossan, a traumatized dog, reinforcing his role as a protector of outcasts.

Interactions with Coo and external pressures drive his growth. Early complicity in bullying quiet classmate Sayako Kikuchi clashes with later courage to defend her, marking his ethical awakening. Determined to help Coo find kin, he ventures into rural Japan, braving media scrutiny and public frenzy that threaten the kappa’s safety.

Confronting escalating crises—invasive reporters, a televised ordeal that triggers Coo’s panic—he faces consequences of his choices. A harrowing escape culminates in Ossan’s sacrificial death, a loss that deepens Koichi’s resilience. Guided by a letter from a fellow mythical being, he aids Coo’s departure, embracing the pain of letting go to honor the creature’s autonomy.

His arc intertwines environmental awareness, humanity’s clash with nature, and fragile bonds across species. The story closes with Koichi bidding Coo farewell, his matured empathy echoed in sustained letters to Sayako, now distanced by family upheaval, underscoring enduring connections forged through compassion and change.