Kosuke and Natsume, two childhood friends.
Now in sixth grade, their relationship has become awkward, and they’ve been avoiding each other lately.
But one day during the summer vacation, they visit an apartment complex that is scheduled to be torn down. This is the place where they grew up.
While they’re playing there, they’re caught up in a mysterious phenomenon. The next thing they know, they’re in the middle of the ocean.
With Kosuke and Natsume on board, the apartment complex drifts through a mysterious ocean...

Will they be able to return to their world?
A summer journey of farewells begins...