"In the sea we live, the sea we protect, and by the sea we go."
Japan, 100 years after plate subductions caused many other countries to be submerged. To save the remaining countries, the coastal cities that had been built up were launched into the seas, and the seaborne routes between them expanded. With this expansion, a need for personnel that could safeguard the seas arose. Women began to seek employment in this new industry, and soon, the "Blue Mermaids," those who keep the sea safe, began to be a source of admiration for many. The childhood friends Akeno Misaki and Moeka China sought to become Blue Mermaids themselves, and together with others who shared their dream, they enrolled in Yokosuka's Marine High School.