The main character of this short story is a boy who loves to imagine. The story takes place in a fictional country where imagination is prohibited. When he tells the authority about his imagination, they arrested him and attempted to re-educate him to stop imaginning. In the climax he escape from the prison, and when he reaches the beach his imagination returns to him. The guards and crowds gathers at the beach and watches the mermaid and the boy setting off the ocean dumfounded.
I believe this short story conveys the message that nothing can stop a man from creating imaginations. Since Osamu Tezuka himself survived through the time of oppression during the WW2 drawing his comic books secretly, so he probably reflected his own experience on the story.
