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.