Nagasumi Michishio, a middle school student, travels to Seto Inland Sea for his summer break. While swimming at Mio Sun Beach one day, Nagasumi experiences a leg cramp and screams for help as he begins to drown in isolation. Miraculously, a mermaid appears and rescues him from his impending death. The mermaid that saved Nagasumi visits him later in the evening, introducing herself as Sun Seto, a member of the yakuza clan's mermaid family. According to mermaid law, a mermaid discovered by humans must be executed, and the only way to save Sun is by forcing Nagasumi to marry her, or face death at the hands of Sun's father and the head of the Seto clan, Gouzaburou. Nagasumi reluctantly accepts the marriage proposal to avoid death. Now married, Nagasumi and Sun must keep their relationship hidden from Gouzaburou's persistent assassination attempts and other obstacles that threaten their union. Despite the difficulties, they grow closer as they adapt to their new life as a couple. Their innocent and genuine love for one another blossoms amidst their unique circumstances.