The premise of this short animation is that the protagonist, a young boy, finds a fish and tries to convince his family that it is a mermaid. However, the state that the boy lives in happens to be against free thought and hands down severe punishments against those who don't comply to the system, a theme reminiscent of Orwell's "1984". Scenes depicting torture and brainwashing follow. Then, at one point we see the protagonist, in a last attempt, studying in order to convince himself that the mermaid shouldn't exist - now a struggle between his logical side and his imagination.
His logical side prevails for a while and his "mermaid" turns into a normal fish. Yet, in a sudden twist, he escapes the totalitarian state with his fish and wades into the sea. Taken literally, it could be seen as suicide, while metaphorically, the act could be seen most familiarly as a bid for freedom. However, I think it's best to take the time the animation was made, the 1960's, into context.
In Japan, the 1960's was a time of extreme societal growth and education was not excluded from that. As having an education became a more attractive idea to the populace, Tezuka must have realized the danger of not being able to stand out in public institutions, as in free thought and creativity.
