Hiyori Yamakawa has grown up observing her father's quirks, finding solace in their shared love for the intricate art of fly fishing. Even after his passing, she continues to frequent their fishing spot by the shore, cherishing the connection to her late father. When her mother announces a remarriage and plans a dinner to introduce their new family, Hiyori, feeling uneasy, escapes to her familiar fishing haunt. There, she encounters the lively Koharu Minagi, who shares her passion for fly fishing. As they bond over their shared interest, Hiyori learns that Koharu is soon to become her stepsister. This unexpected twist marks the beginning of a heartfelt journey as the two girls navigate their evolving relationship and create lasting memories together.