Isamu, a sixth-grade boy, lost his mother in a car accident at a young age. Ever since, he's been unable to forgive those who ignore traffic rules. Each day, he stands at the crosswalk with a yellow flag—his mother's keepsake—guiding his classmates and watching over the signals. Though adults dismiss him and his peers call him a "traffic light fool," Isamu remains committed. When his father announces plans to remarry, Isamu refuses the idea of a new mother. But upon learning that the woman too lost loved ones in a traffic accident, he begins to open up. Through this shared grief, Isamu gradually understands the importance of empathy and realizes that rules alone can't heal a broken heart.