There is a legend that says if someone eats the flesh of a mermaid, they will attain immortality; but this myth contains a darker side, in that the devourer may die or become a monster.

The youthful Yuta has been wandering Japan in search of mermaids not to become immortal, but to break his curse of eternal life. Over five hundred years ago, he ate a mermaid; now, Yuta is in search of a cure, hoping the mermaids know the secret. When he stumbles upon a hidden village, he quickly learns that it is full of mermaids that live in an underground society centered around the murder and cultivation of young human women to maintain their own youth and immortality.

Their most recent victim, Mana, is given the flesh of a mermaid to eat in order to fulfill the recipe toward being a perfect candidate to be eaten. Yuta helps her escape, and they find themselves on a journey to live their lives as normal as they can.