After a pretty mermaid named Sun who happens to be the daughter of a merman yakuza saves him from drowning, Nagasumi has to make a choice: marry Sun or sleep with the fishes.

Nagasumi escorts Sun to a festival so they can get better acquainted before their big day; if Sun's dad has his way, Nagasumi won't live long enough to ask for a second date.

Nagasumi has to tell Sun that it's time for him to return home; while he tries to find the right moment, Sun's tiny bodyguard, Maki the Conch, looks for her own perfect moment to assassinate Nagasumi.

Sun and Nagasumi leave Maki behind and venture out to take in the sights and sounds of the city; when Nagasumi has to confront a couple of bullies, they may wish the tiny terror was still around.

Newly enrolled Sun makes quite a splash among her classmates on the first day of school, but Nagasumi is more concerned with some of the new teachers, Sun's dad and his goons.

Mawari thinks there's something strange about Sun, so she starts investigating. When all signs point to a dangerous situation, Sun might have to reveal her secret in order to protect her friends.