A childhood apartment building becomes a ghost ship, and the show treats it like a character dying in real time. The animation doesn't just look gorgeous, it's doing the emotional work, making every flooded hallway and empty stairwell feel like a memory you're losing while you watch. Summer break becomes the space where kids learn that some things can't be saved, only honored on the way out.

Kousuke and Natsume, two sixth-graders, grew up together like siblings in the same apartment building. Since the death of Kousuke's grandfather Yasuji, though, they have drifted apart. With their old complex about to be torn down, Kousuke's friends pull him into the site to look for a rumored ghost, and he runs into Natsume there. They clash over a camera that belonged to Yasuji, and during the quarrel a sudden, heavy rain engulfs the building. Afterward the children find the complex floating on a vast sea, cut off from everyone else. To find a way home, Kousuke and Natsume will have to mend their friendship. The summer adventure turns strange and haunting, becoming a journey of farewells about growing up and letting go.