Much of Japan’s land mass has been submerged by the rising sea. With the development of marine traffic, the Blue Mermaids, defenders of security at sea, have become an occupation that schoolgirls idolize.

Misaki Akeno dreamed about becoming a Blue Mermaid. She was accepted to a Marine High School and appointed captain of the Harekaze, a Kagerou class destroyer vessel. Her ship sets off for its first training exercise, but her crew runs into some trouble and…