The story takes place in the distant future, when the movement of the continental plates gradually submerged most of Japan's land, and people had to build new cities on the sea. As time went by, the scale of the sea city became larger and larger, and the routes became more and more complicated. In order to maintain order, a new profession called "blue mermaid" was gradually accepted and admired by the public. This glorious profession has also become the goal of many female high school students in the future. The lively, cheerful and optimistic Misaki Akeno (voiced by Natsukawa Shiina), the taciturn and cold-hearted Soya Mashiro (voiced by Lynn), the shy and introverted Tateishi Shima (voiced by Furuki Nozomu), who has a very cute side in front of acquaintances, and the arrogant and poisonous Nishizaki Mei (voiced by Tanezaki Atsumi) who cares about her height very much, can these girls with different personalities realize their ideals?