The main characters of the story are a very imaginative boy and a fish which later on turns into a mermaid. The story takes place in a distant country where imagination is considered to be a crime.
Due to the law of the country, the boy gets arrested. He was tortured everyday until he lost his ability to imagine things. After that he managed to escape from the authorities and went to the beach where he first imagined his fish as a mermaid.
The climax of the story is when the authorities came after the boy only to see that the fish actually was a mermaid, like what the boy had been saying this whole time.
Don't doubt yourself.
I think this is the message of the story.
Even if other people don't agree with you, don't doubt your thoughts and believe in what you know is true.
