Short stories rely on a good structure to make them memorable because otherwise the reader will have finished the story without benefiting from all the elements present in any good story. Short stories do not have time to introduce the reader to the writer's style and they rely on a good structure to get the message across and ensure that the larger theme is shared. Story elements include various features, the most crucial of which are the characters, the setting, the plot, the conflict and the resolution.
In Nadine Gordimer's Once Upon A Time, Gordimer explores the effects of the unknown on a person's outlook. Fear of the unknown becomes the driving force for this family who should be living "happily ever after," except that their fears prevent them from ever relaxing. The characters contribute towards the theme because Gordimer stresses the point that they have normal aspirations and could reflect any family anywhere, making the theme all that more relevant.
The setting may be a Johannesburg street in the 1980s, which is a unique setting, but because of the way Gordimer describes it, it can be recognized universally as people are paranoid and untrusting and some are set in their ways. This setting reinforces that fact that fear and mistrust are often inherent in human nature. Unless people take steps to open their hearts and recognize the needs and desires of others who are different from them, they will never overcome their fears. Note how, in this setting, everything is the same in terms of having high walls and barbed wire, etc. and everyone wants to be the same and, rather than freeing themselves from that, they perpetuate it and become a part of it. Hence, the setting develops the theme of fear of the unknown as people would rather endure the unpleasant side of living in this neighborhood rather than actively trying to do anything about what is causing the irrational fear. Taking precautions against burglaries and so on is not unwise but taking it to extremes is what Gordimer is warning her readers about.
The conflict in this story develops the theme because it is the reason why the family find it necessary to keep intensifying security. How can they ever be safe when they hear so many stories and every precaution they take is never enough? Even the cat can effortlessly scale the boundary wall so how will it deter a would-be armed robber?
The plot of any story has its own elements as it gathers momentum towards a climax after which the outcomes become apparent. In Gordimer's story, the rising action is evident in all the steps the family take towards ensuring its safety. The climax is the boy's tangle in the barbed wire and the outcomes are left to the reader's imagination as the "bleeding mass of the little boy" is freed from the wire and carried into the house. The use of the word "it" to describe him suggests a sinister end to this story. The resolution then confirms the family's fear of the unknown. However, it is hardly the "unknown" that they were expecting.