Childhood friends Akeno Misaki and Moeka China want to become Blue Mermaids, a new female force established to protect the remaining coastal towns after many countries have sank into the sea. Together with others who shared their dream, they enrolled in Yokosuka's Marine High School.
Long ago, all humans lived in the ocean. But some who longed for the land abandoned the ocean, casting off the special raiments granted to them by the sea god to adapt them to life under the sea...
Though mankind was divided into sea dwellers and land dwellers, each with different ways of thinking, they were all still fellow humans, and they continued to maintain contact as time went on.
Sakishima Hikari, Mukaido Manaka, Hiradaira Chisaki, and Isaki Kaname live in an undersea village, while Kihara Tsumugu lives on land.
When these middle school second-year land and sea dwellers meet for the first time, their hearts begin to undulate like the tides.
The youthful fantasy tale of these young people's lives unfolds in a strange and beautiful world.
Squid Girl has come to the land from the depths of the sea to conquer humanity for its pollution of the ocean. Unfortunately she ruins the first house she uses as an invasion base and has to work to pay for repairs. Of course, she can't overcome the Aizawa sisters who manage the house, so who knows whether she can subjugate humankind.
At private Mihama Academy, five girls who seem normal attend the school due to "certain reasons." A sixth student "with reasons" named Yūji Kazami transfers in. Yūji has an ego with such darkness that he does not fade in comparison to the five girls, who are so different from the norm. As they each meet each other, they deepen their understanding of one another. Through the small bits of progress they make, each of their hearts will be affected. When Yūji arrives, a story of girls who are trapped by their pasts begins.