The main character is a boy who loves to imagine things.
One day, he finds a fish, who turns into a mermaid. I think that the time they spend together shows the innocence and joy he has with just being with her. However, no matter how much he enjoys it, he cannot stay there forever. So, he wishes to return, with the mermaid, to where his village, or society is.
This is where the conflict arises, because the society he lives in doesn't allow him to use his imagination, and uses different methods to make him forget about the mermaid, and see the fish only as a fish, like the rest of the people in the society. The boy desperately tries to remember, but to his horror, finds out that he is unable to remember.
In the end, he takes the fish and returns to the ocean. As many people watch from the shore, the boy himself enters the ocean and follows the fish, which turns back into a mermaid. I think that the boy decided to leave society, and became a merman, free from the regulations of society.
I think that the theme for this animation is the defiance of common sense. As we grow, we learn how to live in a way to contribute to our society and are filled with what the world calls common sense. Those who cannot adjust tend to be cast away or considered to have no value. I think that Tezuka Osamu wanted to tell us that we have to be careful to not let what others think be our number one priority.
