A century after cataclysmic movements in tectonic plates submerged the majority of Japan, the country today flourishes as a marine nation. Japan's coasts are lined with high-functioning cities, and well-developed marine routes are guarded by specialist navy ships. The maritime business is thriving, and teenage girls are eager to join the Blue Mermaids, a group of highly talented women who devote their lives to ocean conservation. Childhood pals Akeno Misaki and Moeka China are reunited in Yokosuka Girls' Marine High School after some months apart. They are joined here by others who share their desire to become Blue Mermaids. Soon after, training drills begin, with Akeno and Moeka given command of their own ships and crew. Everything is going swimmingly for Akeno.