One summer event would bring junior high school student Michishio Nagasumi’s peaceful life crashing down.

While visiting his hometown, he almost drowns but Seto San, a mermaid, saves his life. However, her family is part of a merfolk mafia, and they abide by a strict rule: If a mermaid is discovered by a man, either he dies, or the mermaid dies. The only exception is if the man becomes a member of the family. When Nagasumi is threatened to either die or join the family, he comes to the conclusion that he must marry San.