Realizing he's gay, Shun walks out on his own wedding and flees to the island where his grandmother lives to escape his family. There, with the excuse of writing books, he befriends Mio, a boy who has lost his family. Before Mio is sent to an orphanage, he kisses Shun, promising to return. When he does three years later, Shun is conflicted, remembering his own family's reaction to him coming out, but they ultimately make future choices together.
It deals with isolation, loneliness, and living honestly.
Some other commenters complained that this seemed randomly thrown together. I disagree, although the transitions could have been clearer. There were a few times I had to stop and say, "Oh, we've jumped ahead in time," but the story itself doesn't bother me: Boy moves to island, boy meets other boy, other boy leaves island, other boy comes home and confesses his love three years later, they dance around each other, former fiancee makes boy confront reality, boys affirm their love, boys leave the island together.
Umibe no Etranger