The opening of the story seems to imply that the story which follows is a story for children, a bedtime story at least, since this is the story the speaker tells herself when she cannot fall back to sleep after having been startled and kept awake by what sounds like footsteps, though it was just her house settling atop compromised foundations.
The author, with the acute senses of one who feels abject fear, watches her bedroom door as though she expects someone with bad intentions to burst through it at any moment. She thinks of how a woman was murdered in a house only two blocks away from her own home just last year and how she has taken no extra precautions to be sure that her home would keep an intruder out: there no bars over her windows, which are only made of thin glass, for example.
Fear and hyperawareness set up the story in which the husband and wife "who loved each other very much and were living happily ever after" begin to try to secure their home against the possibility of invasion and end up not only becoming prisoners in a prison of their own design but also end up catastrophically injuring their son as a result of their efforts. The opening of the story suggests that the possibility of securing oneself against all harm is, at best, a fantasy: an alluring fiction that gives us a feeling of control—but no actual control. In the end, however, security and fear can have unintended and disastrous consequences.