about the mermaid

I think the main character is the boy, and he wanted to share to everyone what he saw and brought from the ocean, which is the mermaid. However, everybody except the boy couldn't see the mermaid. To them, it was just an ordinary fish. Everyone thought it was just his imagination, and that was the problem, because his country didn't allow people to imagine. He just wanted everyone to know about the mermaid, but he couldn't make himself understood. He was put in a jail and was made to forget about the mermaid. At the end, he forgot about the mermaid, and released the fish, but then, the fish turned into a mermaid again and the boy went into the ocean with the mermaid to get freedom. I think Tezuka wanted to tell us that imagination is not a bad thing, and everyone can imagine what they want unless it wouldn't do harm to other people.