"Kappa no Kaikata" centers on the daily life of Kazuma Kawaguchi, a young man who unexpectedly becomes the caretaker of a kappa named Kawauso. The story begins when Kazuma, a college student living alone, discovers a small, injured kappa in the river near his home. Despite his initial reluctance, he decides to take the creature in, naming it Kawauso after its species. The narrative explores their evolving relationship as Kazuma learns to care for the kappa, navigating the challenges of raising a mythical being in a modern human world.

Kawauso, though small and childlike in appearance, possesses the traditional traits of a kappa, including a love for cucumbers, a water-filled dish on its head, and a mischievous yet endearing personality. Kazuma’s life becomes increasingly intertwined with Kawauso’s as he adjusts to the kappa’s needs, such as providing a suitable aquatic environment and managing its curiosity about human habits. The story balances humor and warmth, depicting Kazuma’s growing affection for Kawauso and his efforts to protect it from the outside world.

The series introduces additional characters who contribute to the dynamic, including Kazuma’s childhood friend, Mayuko, who becomes involved in caring for Kawauso, and other mythical creatures that occasionally cross paths with the duo. These interactions often lead to comedic or heartwarming situations, highlighting the contrast between the mundane human world and the fantastical elements introduced by Kawauso and its kind.

The plot progresses episodically, focusing on small, self-contained stories that emphasize the bond between Kazuma and Kawauso. Themes of responsibility, coexistence, and the blending of the ordinary with the extraordinary are central to the narrative. The series avoids heavy drama, instead opting for a lighthearted and slice-of-life approach that underscores the charm of its premise. Through its exploration of Kazuma’s journey as a caretaker and Kawauso’s adaptation to human life, "Kappa no Kaikata" offers a unique and engaging portrayal of interspecies friendship.