A pleasant, pretty, and sometimes touching movie. Understated for such a melodramatic plot. But not anywhere near one of Ghibli's best.
Umi has a lot of responsibilities. Though she's only in high school, she needs to take care of her family, and the boarders in her house. Her father died in the Korean War, and her mother is studying in America. With so much responsibility, she doesn't have much time for herself.
In the background is Japan of the 1960s. It's transitioning into a more Western, perhaps more modern country. It's about to host the Olympics - so Japan's officials want to put a particular face forward to the rest of the world.
The "Quartier Latin" isn't something I completely understand. It seems to be semi-officially attached to Isogo High School, the school Umi attends, housing all the school's clubs. Inside, students observe the stars, debate ancient philosophy, publish a newspaper - all manner of scholarly activities. It seems like an awesome place. The film treats its residents kindly - though their interests are portrayed as a bit silly, and ineffective, we like everyone inside. In an American movie, I think they'd be treated as nerds and outcasts, but I don't get the sense the Quartier Latin is looked upon that way. Maybe anti-intellectualism is an American phenomenon.
The building itself is old, and falling apart, and the entirely male student body inside have left the inside in shambles, cluttered with dusty books, and papers. And so, the club house - a relic, and symbol, of a past that Japan is trying to move beyond - is marked for demolition. Trying to save "Quartier Latin" and, symbolically, a piece Japan's past, is one of the two driving forces of the movie
The other is the romance between Umi and Shun, a student who is involved in the Quartier Latin' journalism club. He publishes a secret letter of admiration for Umi in his paper, and she is drawn to him after she reads it. His quest to save the club becomes hers too. We see them subtly growing close to each other, and it's pleasant to see their unstated affection for each other. Their feelings for each other are never stated outright, until Shun makes the discovery that the two of them might be brother and sister. Then, after acknowledging their feelings to each other, they must struggle with what fate seems to have handed them.
The plot is melodramatic, even the characters in the movie know that, but it's played in an understated way.
This is the kind of movie in which things work out well in the end. I found the resolution of both plots touching even though I knew how things would work out. In large part the emotional effectiveness is due to the music, which is reminiscent of the music from earlier Studio Ghibli movies.