Tatsuhiko, a quiet high school student in the coastal city of Karatsu, spends his days fishing and drifting through adolescence. Just before summer vacation, Yoon-seul, an exchange student from Busan, joins his class. As they grow closer, Tatsuhiko gradually discovers the heartfelt reason she chose this small Japanese town. However, an innocent remark unexpectedly hurts Yoon-seul, and she abruptly returns to Korea. Unable to let go of his regret, Tatsuhiko makes a life-changing decision—to cross the sea in search of the girl he never truly had the chance to understand.