Kousuke and Natsume have been friends since childhood, but as time goes on, the relationship between them seems strained as they keep avoiding each other. During a visit to a housing complex scheduled for demolition during the summer vacation, they are confronted with a mysterious phenomenon that transports them into an ocean where their previous world has disappeared. As they come to terms with this new reality, they embark on a journey of self-discovery and friendship as they try to find a way back home.
The apartment complex where the film is set is modeled after the original designs of the Hibarigaoka Danchi, which was one of the largest housing complexes in Tokyo during its time.