Nagasumi Michishio is saved from drowning by a mermaid, San Seto but, according to mermaid law, if a human sees a mermaid's true form, both have to be killed and the only solution to the problem is for them to marry.
But can they keep San's secret from being found out and can Nagasumi keep himself from being killed by San's Father and his Yakuza associates all at the same time?