A first-year junior high school student named Makoto Natsume living in Chōfu, Tokyo, has his ordinary life turned upside down when a classmate named Aki Hayama suddenly confesses her love for him. The confusion deepens when Aki explains that her grandmother once told her she would meet a boy named Makoto Natsume in the future, a prophecy that means little to the bewildered Makoto who is certain they have never met. The connection between them begins to take shape when Aki shows him a mysterious painting of a fireworks display, and Makoto agrees to help her find the specific boy from her grandmother's story.

Convinced the painting holds the key, the two travel together the following year to the Nagaoka Festival Grand Fireworks Show in Niigata Prefecture, one of Japan's three major fireworks festivals which draws hundreds of thousands of visitors annually. Makoto is characterized as shy, lacking in self-confidence, and awkward in social situations, while Aki is the popular and talented class president, admired for her beauty and abilities. Despite their contrasting personalities, the mystery of the fireworks painting binds them together as they find themselves at the mercy of fate and a limited amount of time to make a life-altering decision. As the story unfolds, the grand summer festival in Nagaoka becomes more than just a backdrop, serving as the emotional core where past promises, present feelings, and future choices collide under the night sky illuminated by fireworks.