The theme of this short story is that our fears of other people, especially people who seem different from us, can destroy us. More specifically, Gordimer, writing in 1989, shortly before apartheid's collapse in South Africa, uses her story to attack apartheid as a sick system that hurts everyone involved. She illustrates in her tale that fear of the other not only harms the groups that are feared; it also can destroy the people who react out of excessive fear.

In this story, a family living in a privileged, all white enclave in South Africa become increasing frightened by stories of blacks robbing neighborhood homes:

But every week there were more reports of intrusion: in broad daylight and the dead of night, in the early hours of the morning, and even in the lovely summer twilight ...

When the family sees their cat leap to the top of the tall wall that they have constructed to stay safe, they decide the only way to be truly secure is to top the wall with shards of broken glass and razor wire. 

One night, the mother reads her son a fairytale about a prince who scales a high castle wall. The next day, wanting to be like the prince, the boy tries to scale the wall surrounding his house. He gets caught on the broken glass and razor wire and dies. 

Gordimer, a communist dedicated to breaking down racial barriers in her homeland, liked to quote philosopher Antonio Gramsci:

The old is dying and the new cannot be born; in this interregnum there arises a great diversity of morbid symptoms.

This story shows that one of the chief qualities of these "morbid systems," excessive fear, is highly destructive. It doesn't protect families; it destroys them. This family tried to build a high-walled fairytale castle where they could retreat and be safe. However, they were living in the actual world, where reality intrudes and disrupts our fantasies.  They would have been better off to have been less fearful and more open to the blacks.

The most important theme in Nadine Gordimer's "Once Upon a Time" is Fear of "the other."

This fear can be perceived or real, and the attainment of the perfect and secure life can lead to one's own destruction.

The family's perceived fear of Black people originates from rumors and news reports which is then confirmed through reports of home invasions in their neighborhood.

When each step in their plan does not meet their need for the perfect, secure life, they take more drastic measures.

Thus, Fear of "the Other" led the family to join a neighborhood watch group, install a home security system, limiting the hours of work and location of their Black servants, and, finally, to the installation of barbed wire fencing that led to the Death of their son as he climbed it after learning about the heroic deeds of the Prince in Sleeping Beauty.

The family's wealthy status and privilege may have served as obstacles to acceptance of their neighbors.

The implication is that fear of the unknown and its associated changes seems so threatening that people will go to extremes to preserve the status quo.