Middle school students Keisuke Isobe and Koume Satou live in a seaside town where not much happens. But when Koume's crush breaks her heart, they find themselves in a very strange situation. She starts a "no-strings-attached" relationship with Keisuke, who she had turned down before. Both of them find comfort in the other to fill emotional gaps in their lives. But being "friends with benefits" gets harder when real feelings start to grow and the effects of their relationship start to show on themselves and the people around them. Umibe no Onnanoko looks at love, relationships, and the emotional price that will be paid because of a choice made by two teenagers in a harsh way.