"Rainbow Fireflies" is set in the late 1960s in a rural Japanese village and centers on a group of sixth-grade students navigating the challenges of adolescence, family struggles, and societal changes. The protagonist, Yuuta, moves to the countryside to live with his grandparents after his mother falls ill. The story unfolds during the summer break, capturing the essence of childhood innocence and the bittersweet transition into adulthood.

Yuuta befriends a group of classmates, each dealing with their own personal hardships. Among them is Aoi, a spirited girl who dreams of becoming a singer despite her family's financial struggles, and Tetsuo, a boy grappling with the absence of his father, who works far away in the city. The group spends their days exploring the countryside, engaging in playful adventures, and forming a tight-knit bond. Their shared experiences are punctuated by moments of joy, such as catching fireflies by the river, and moments of sorrow, as they confront the realities of their families' struggles.

The narrative takes a poignant turn when the children decide to organize a fireworks festival to lift the spirits of their community, which has been affected by economic hardship. The festival becomes a symbol of hope and resilience, bringing the villagers together and allowing the children to express their creativity and determination. However, the event also forces them to confront the fragility of their dreams and the inevitability of change.

As the summer draws to a close, the children face the prospect of separation, with some families considering moving away due to financial pressures. The story captures the fleeting nature of childhood friendships and the enduring impact of shared memories. Through its exploration of themes such as family, friendship, and perseverance, "Rainbow Fireflies" paints a heartfelt portrait of a pivotal moment in the lives of its young characters, set against the backdrop of a changing rural Japan.