With her 17th birthday coming up tomorrow, Nanami Rukia moves to a town by the sea. For some reason, she is attracted to her classmate Kurosuna, who is super handsome but has a terrible personality... However, he only seems to be interested in the "legendary mermaid."And then, on her birthday, Rukia's mother, Lucia, tells her that she is the daughter of a mermaid who protects the seven seas?!