A totalitarian world punishes imagination itself, and one boy's refusal to stop believing in what he saw costs him everything. The mermaid is real to him because he imagined her into existence, and the state cannot tolerate that kind of freedom. They take his imagination as punishment, turning a fairy tale into an act of resistance.

A boy lives in a fictional country where imagining things is against the law. When he saves a fish, it turns into a mermaid, and everyone who sees it is astonished. The authorities arrest him for imagining such nonsense, and as punishment they take his imagination away. Tezuka Osamu's fantasy film portrays the frightening side of a controlled society and the idea of escaping it. It treats imagination as a gift that lets people soar, and it criticizes an education system that crushes individuality, a concern that echoed criticism of Japan's intensely competitive examinations at the time.