According to an ancient tale, eating mermaid's flesh can confer immortality. Yuta inadvertently consumed mermaid meat 500 years ago. For millennia, he travels across Japan, seeking to locate a mermaid who can turn him back into a regular person. When he eventually locates one, he discovers that she and her buddies have been rearing a girl to be their meal, so that they may eat her and assume her young appearance. That is how mermaids maintain their youth. Yuta kills the mermaids and saves her, but she has already consumed part of the mermaid meat. Yuta isn't too sad that he had to slay the mermaids. Yuta's previously lonely existence has come to an end now that he has met a buddy.