Ever since the death of his grandfather, Yasuji, Kousuke Kumagaya has grown distant from his close friend, Natsume Tonai. Having grown up together like siblings in the same apartment building, the two now find themselves at odds with one another due to Natsume's inability to let go of the past. This rift is further exacerbated by the news that their former complex will soon be demolished. Hearing about a ghost that is supposedly haunting the old complex, Kousuke's friends drag him into the demolition site to look around. When Kousuke finds that Natsume is also there, they get into an argument over an old camera that once belonged to Yasuji. This results in Natsume running off with it, but just as she is about to fall off the rooftop, a torrential downpour envelops the entire building, flooding the ground around it. The young children now find themselves drifting at sea with the apartment building acting as their ship. Unable to communicate with the rest of the world, Natsume and Kousuke must reconcile their differences if they want to stand a chance at finding their way 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.