My Girlfriend is a Mermaid!?

A young man returns to his rural hometown for the summer and encounters his childhood friend Ion — who is now a mermaid. Shortly after, he meets Petakko, a mermaid with no memory of who she is, and Rin, a priestess who watches over mermaids but has a split personality. The encounters draw him into an old legend surrounding mermaids, with Ion at its center. A strange and eventful summer begins.