Legend holds that consuming mermaid flesh grants immortality, yet this myth has a darker side: the eater may die or turn into a monster. The young Yuta has been wandering Japan looking for mermaids, not to gain immortality, but to break his curse of eternal life. More than five hundred years ago, he ate a mermaid; now, Yuta seeks a cure, hoping the mermaids know the secret. When he stumbles upon a hidden village, he quickly learns it is full of mermaids living in an underground society centered on the murder and cultivation of young human women to maintain their own youth and immortality. Their most recent victim, Mana, is given mermaid flesh to eat to fulfill the recipe for becoming a perfect candidate to be eaten. Yuta helps her escape, and they find themselves on a journey to live their lives as normally as they can.