Summer Days with Coo

Approximately 200 years ago, the kappa's father is killed after a dreadful encounter with a samurai. As he cries, the ground opens up and swallows him whole. This brutal event marks the kappa's origin of tragedy.

In present-day Tokyo, Kōichi Uehara faces peer pressure and joins his friends in harassing Sayoko Kikuchi, who they cruelly nickname ugly. The bullying strains their social circle and pushes Kōichi toward a deeper curiosity about Sayoko. This incident sets the stage for the later discovery of the kappa.

During a muddy ditch incident, Kōichi discovers a stone containing a sleeping kappa. He brings the stone home, and the creature emerges into the Uehara household. The family is awestruck by the sudden presence of a real kappa.

The Uehara family welcomes the kappa into their home, and he is named Coo after he forgets his own name. Coo quickly warms to the family, especially Kōichi. Hitomi, Kōichi's sister, initially resents him but gradually comes to accept him.

The family's dog Ossan reveals he can communicate with Coo via telepathy. He warns Coo to keep Ossan's ability a secret from the Ueharas. This creates a quiet tension and trust within the household as they navigate their new reality.

Coo is spotted by a couple at night, and news of a kappa sighting starts spreading across Japan. Paparazzi camp around the Ueharas' house, invading their privacy. The sudden attention tests the family's tolerance and Coo's need for safety.

To learn more about his origins, Kōichi takes Coo on a holiday to the countryside. They stay in a rural house, where Coo communicates with a Zashiki-warashi who believes Coo may be the last kappa. The encounter deepens the mystery and strengthens their bond.

Upon returning home, journalists ambush the Ueharas and snap pictures of Coo. The family feels the pressure of the media spotlight and contemplates how to protect their privacy. The invasive attention foreshadows further conflicts.

On the talk show, a cryptid expert claims the arm is connected to the kappa's origin and appears to be the kappa's arm. Coo recognizes the arm as his father's, cries, and uses his powers to destroy lights and cameras, fleeing with the arm aboard Ossan. In the ensuing chase, Ossan is struck by a car and dies.

Rain begins to fall and a water-elemental dragon appears in the sky, convincing Coo to continue living. He is rescued shortly afterward.

Kōichi grows kinder to Sayoko thanks to Coo's presence, and she reveals she is moving away due to her parents' divorce. He defends her honor, strengthening their bond and his own resolve to stand up for others.

Back home, Yukari informs that Coo is leaving after receiving a Yokai handwriting letter. The Ueharas bid him farewell, with Yasuo distracting paparazzi as Kōichi escorts Coo out in a box to ship him off. They stop by Kikuchi's apartment to show Coo and promise to write.

Coo arrives in Okinawa, where a kijimuna who saw him on the news becomes his new guardian and promises to teach him how to change his appearance. He tells Coo to call him Ossan. Coo ventures into Okinawa's lush greenery with his father's arm, vowing to continue seeking more kappas and to someday reunite with the Ueharas.