Having inadvertently eaten mermaid flesh 500 years ago, Yuta has achieved his dream of eternal life. But after losing so many of his loved ones to the natural course of aging, he's grown discontent and wants to become mortal once again. Setting out to find another mermaid in hopes that she can tell him what to do, Yuta meets Mana, an immortal girl looking to become human, and they set out on their quest together.