The characters in this film are a boy with great imagination, a fish/mermaid, the government, and the boys parents.
Setting
It was a time where having your own thoughts, own opinion, or your own imaginary thoughts were not allowed. Everything was controlled by the government and if you did not obey them, they would torture you no matter what.
What do they want?/What is the problem?
The boy wanted people to believe that he was seeing a mermaid when all the others were saying that they only see a fish swimming, not a mermaid. As the boy is claiming that he is seeing a mermaid, people around him thought that he was crazy. In order to make him 'normal', the government forced him to forget the mermaid in his mind by torturing him in two ways. The government did not allow the boy to think freely. They wanted the boy to believe in what they said.
What is real and what is fantasy?
The real thing in this film is that having your opinion and expressing it was prohibited. Therefore the boys imagination of seeing a mermaid in the tank was a fantasy. So, the government controlling the boy was real. I guess the boys mind was being brainwashed not to believe in his fantasy.
The end of the story
I think the boy died in order to be free in heaven.
What the creator wanted to talk about?
I think the creator wanted to tell the audience that judging others imagination and thoughts should not be restricted.
