Tōko Fukami's family runs a glass-working business in a small seaside town. She hangs out with her four best friends at a cafe called "Kazemichi." During the summer break of their senior year in high school, they meet a transfer student named Kakeru Okikura, who claims that a voice from the future talks to him, and that it's led him to Tōko. His arrival sets off a series of events that will make their final summer together one full of hope and heartache.
Yuichi Aizawa returns to this city after seven years. In his childhood, he often visited it, but something happend in the past which caused him to block out his memories of the events of seven years ago. On the day after he gets settled in, he meets a girl carrying a backpack with wings on it. Her name is Tsukimiya Ayu and he begins to recall the memories he had lost.
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.