Since Nagasumi saw Sun as a mermaid when she saved him from drowning, by mermaid law they must be executed. To save Sun's life her Yakuza family reluctantly agrees they should marry. However, her father dislikes this and tries to get Nagasumi killed, while Sun's mother -- who supports the marriage -- protects him. Since they will be living together, Sun will also attend Nagasumi's high school. Despite all of the problems they experience, Nagasumi and Sun fall in love.
http://www.comic-con.org/wc/wc_anime.php
