Do you see the point of the story, Nathan? We all cut parts of ourselves away, but we never lose them. Things stay with us-souvenirs with memories attached. We can't always choose what to keep, what to throw away.
Nathan's parents devised cautionary tales for him and his sister-gruesome stories about predatory cars racing along the "Big Street" at one end of their neighborhood, or dope fiends lurking in the woods behind their house and ready to plunge hypodermics into the skin of foolish young trespassers. These stories served their purpose during Nathan's gullible childhood, essentially constructing an invisible fence around the yard and keeping the boy close to home where he'd be safe.
Such barriers are not so easy to discard in later life. As an adult, Nathan no longer believes his parents' stories, and yet they still confine him. He lives cautiously, avoiding serious relationships, avoiding risk. But despite his efforts, something from his parents' cautionary tales threatens to creep beneath that invisible border...and the enclosed yard might not be as safe and secure as it always seemed...