Since childhood, Hiyori Yamakawa has been familiar with her father's eccentricities, but one activity she always enjoyed doing with him was fly fishing. This complicated yet rewarding method of catching fish brought her much joy, even after her father's sudden death.
Three years later, Hiyori's mother decides to remarry and arranges a dinner with her fiancé, his daughter, and Hiyori to celebrate. To pass the time, Hiyori goes to her usual fishing spot when she meets Koharu Minagi, an energetic girl. The two quickly become friends and Koharu takes an interest in fly fishing. It turns out that Koharu is Hiyori's future stepsibling, and the two of them start to make memories together as they foster their sisterhood.