Michishio Nagasumi's life couldn't be any more normal. In an odd twist of events, during his summer vacation he ends up almost drowning in the sea. Luckily, the cute mermaid Seto San appears to save him. However, San is from a yakuza mermaid family and according to their law, if a human is to catch sight of a mermaid, either he or the mermaid must die. The only other way is for Nagasumi to be taken in as a family member, marrying San. In attempts to save both of their lives, Nagasumi asks for San's hand in marriage. Nagasumi's summer vacation reaped more then what he would expect, as he must now protect San from others finding her secret out.