Mōta spends his childhood in a mountainous rural area as the son of a local school teacher. Monthly visits to a city school expose him to persistent bullying by children who mock him as a country bumpkin, resulting in significant social isolation. This profound loneliness fuels his longing for companionship.

His solitary existence shifts when he encounters a supernatural entity named Rain Boy near a bridge. Resembling an umbrella and accompanied by a personal rain cloud, Rain Boy becomes Mōta's first real friend. This being interacts physically with the world, suggesting it may be a tsukomogami – an object inhabited by a spirit. Visible only to Mōta, their bond is unique and private.

Rain Boy aids Mōta against his bullies, notably covering them in mud for retribution, and later intervenes to save the local school from a destructive fire, proving its tangible nature. Their friendship deepens through these shared events. During their childhood, Mōta promises Rain Boy a pair of rain boots in exchange for three wishes, though he later forgets this vow.

Mōta's family eventually relocates to the city, severing his ties to the rural area and his encounters with Rain Boy. The move and the passage of time cause him to completely forget both the promise and Rain Boy's existence. Years later, as an adult with his own wife and child, a chance event triggers Mōta's memory of Rain Boy and the unfulfilled promise. Driven by this recollection and a sense of obligation, he returns to his hometown. He seeks out the bridge where they first met, bringing the rain boots in an attempt to honor the long-overdue agreement. The narrative concludes as Mōta confronts the consequences of his forgotten promise and the elusive nature of his childhood companion.