In a seaside town where very little happens, middle school students Keisuke Isobe and Koume Satou live a rather dull life. But when Koume's crush breaks her heart, their situation becomes quite unordinary. She starts a "no-strings-attached" relationship with Keisuke, whom she had previously rejected, both finding solace in the other in order to fill the emotional voids in their lives. However, being "friends with benefits" becomes complicated when real feelings begin to develop, as the consequences of their relationship start to take their toll on those around them and themselves.

Umibe no Onnanoko takes a harsh look at love, relationships, and the emotional price that will be paid as the result of a decision made between two teenagers.