The lost thing is the film which takes place in the town near the beach. One day, a boy was collecting bottle tops by the seashore. There, he finds a huge, strange-looking creature. It seemed kind of lonely, and they became friends. The boy tries to find where the creature belongs to, and travels many places. The people around the creature seem unable to see the creature itself. Finally, the boy finds the world it belongs to and they say good-bye and the film ends.
I think this film is trying to tell us that nowadays we are forgetting our imagination and therefore focusing too much on the real life. It is sometimes important to look back on the childhood and feel nostalgic about the imagination we have done.