Zachary is adrift in the world. He has cut himself off from most of his past, dropped out of high school, broken up with girlfriend, lost touch with his best friend who moved away, and seems content in his discontent. He is also angry at the world, an anger that often explodes in violence. When a random act of violence enters his life—his father is shot during a car-jacking—his life begins to spin out of control. When it appears that the person arrested for shooting and paralyzing his father will go free due to a lack of evidence, Zachary becomes obsessed with seeking vengeance. Retrieving a gun he acquired years ago, Zachary is determined to see justice done.
Cadnum says that like his earlier novels Calling Home and Breaking the Fall, Edge is based on a true event. In those books, however, the idea sprang from an article he read in the newspaper. As he explained to Shoemaker, the events in Edge happened closer to home:
A friend of mine was shot and badly injured in a carjacking, and I understood the hunger some people have to take revenge— to get a gun and bring justice to the man who did this to my friend. That anger and sorrow was the seed that eventually grew into my novel.... I have a great hunger to know why people do such violent things, and to discover how we can live with the aftermath.