Nagasumi Michishio nearly drowns during a summer trip to the Seto Inland Sea, only to be rescued by Sun Seto, a mermaid and daughter of a yakuza-style mermaid clan. Under mer-society law, a human who learns a mermaid's identity must die, unless the two marry. Forced into an engagement, Nagasumi returns home to find Sun transferring to his school, with her overprotective father Gozaburo infiltrating the faculty to keep watch over the couple.