During his summer break, Nagasumi Michishio almost drowns at a beach in the Seto Inland Sea, but is rescued by a mermaid named Sun Seto, who happens to be from a yakuza family. According to mermaid law, Sun must be executed for revealing her identity to a human, but to spare her life, Nagasumi agrees to marry her.
However, they must keep their marriage a secret from everyone, including Gouzaburou, Sun's father and leader of the Seto clan, who constantly threatens Nagasumi's life. Despite the challenges they face, Nagasumi and Sun's love for each other grows.