It's 1963. Every morning Umi raises the signal flags up on top of Poppy Hill, in memory of her father who passed away in the Korean War. Little does she know that each day the flags are seen by Shun, her upperclassmen, from his father's tugboat as it passes by. Shun and Umi meet in school, and as they band the student body together to save the beloved clubhouse from plans for demolition in preparation for the Olympics, gradually Shun and Umi grow to be more than just friends. Unfortunately, the question of Shun's parentage, and that he may be Umi's brother, forces them apart. However, Umi is not to be swayed from either the clubhouse, or from her feelings for Shun, and the truth about Shun is revealed the same day the fate of the clubhouse is revealed.