This was sad and a little difficult story for me,but I'm going to write what I supposed.
The main character is a boy who likes to imagine.One day,he found a small fish at the beach and the fish became a mermaid only in his mind.They came to be close and had a good time in the sea.He brought her to his home and talked proudly about the mermaid in the tank to his parents.But what his parents could see is not a mermaid but a small fish.It was only him who could see the mermaid.The fancy was prohibited in his country so he was captured and forced to forget her.Finally,he fled from the prison.He came to be able to see the mermaid again and left to the sea with her.
I think his imagination came true at the end.It is impractical,but this story makes us think about the freedom to imagine.Though people can imagine or think freely in almost every countries now,I felt it is more important than we think of.