Kousuke and Natsume have been friends since childhood, but their friendship appears to be strained as the two sixth graders avoid each other. During their summer vacation, they go to a housing complex that is about to be demolished. They have many memories of growing up there, but while playing, they become engulfed in a mysterious phenomenon. When they regain consciousness, they see an entire ocean in front of them, as the housing complex has drifted into a mysterious sea, containing Kousuke, Natsume, and others. Will they be able to return to their former lives? A journey to say farewell to summer begins...