Plot
Kosuke Kumagaya and Natsume Tonai are 11-year-olds who grew up like siblings. The relationship between the two began to be strained after the death of Kosuke's grandfather, Yasuji Kumagaya.
One day during summer vacation, Kosuke and his classmates of Kamo Elementary School, named Taishi, Yuzuru, Reina, and Juri, sneak into Kamonomiya apartment complex, known by the kids as the "ghost housing complex", that has been slated for demolition. This housing complex happens to be a house full of memories where Kosuke and Natsume grew up, alongside grandfather Yasuji.
There, Kosuke unexpectedly encounters Natsume, who tells him and the others about the existence of a mysterious boy named Noppo living in the complex. Kosuke and Natsume start to argue, the group later chips in, then heavy rain ensues. After the rain stops, the housing complex is suddenly drifting in the middle of the ocean.
With no one in the building except them, the kids attempt to survive on their own, by exploring other nearby buildings also floating in the ocean, on the search for food. During their time together, tension in the group rises, as injuries and arguments happen.
After their building crashes into another, a hole is made at the bottom, and water starts to flow inside. The group must leave the building and go out in the ocean. Using objects they find in the apartment, they build a raft.
Everyone boards the raft except Noppo, who pushes the boat away from the apartment where he decides to stay, to the group's surprise. Natsume refuses to leave him behind, and swims back to the building. Kosuke follows her into the water, but fails to catch up to her and the building.
The group is separated: Noppo and Natsume are on the roof of the apartment complex, whereas the rest are in the raft. Later, the raft is approached by a floating Ferris wheel. The kids use this huge wheel to reel in the building Noppo and Natsume are on, with the help of a mysterious women who appears from this Ferris wheel.
The group reunites on the initial building, and reaches a mysterious shore, where they see other kids waving at them. The group thinks of getting off the building and onto the shore, but Noppo stops them and leaves alone, saying they won't be able to return home if they leave with him. The group lets him go, and are somehow led back home, by a mysterious way only Noppo seems to know. They return to their normal lives, and Kosuke and Natsume find themselves closer than ever.