Although Kousuke and Natsume have been friends since they were young children, their friendship has grown strained as a result of their continued avoidance of one another. They go to an apartment building that is going to be torn down one day while on summer break. The location contains many memories for them because they grew up there, but as they play, they unexpectedly become involved in a puzzling occurrence. When they come to, the housing complex has drifted into a mystical sea with Kousuke, Natsume, and others in it, and they see an entire ocean before them. Can they get back to the world they left behind? A voyage to bid summer adieu begins...