Since childhood, Hiyori Yamakawa has observed her father's many eccentricities, yet one activity she has always cherished doing with him is fly fishing—a complex but rewarding way to catch fish. Even after his sudden passing, Hiyori continues to visit the coast in her free time, enjoying the comfort that her father's keepsake provides. Three years later, Hiyori's mother decides to remarry and organizes a dinner with her fiancé, his daughter, and Hiyori to mark the occasion. Feeling anxious and wishing to kill time, Hiyori heads to her usual fishing spot, where an energetic girl named Koharu Minagi suddenly appears before her. The two quickly get to know each other, with Koharu developing an interest in fly fishing. However, their encounter is more than mere coincidence: Koharu turns out to be Hiyori's future stepsister, marking just the start of the cherished memories they will reel in as they build their sisterhood.