Chapters 1-5 of Unbroken give the necessary background of Louis Zamperini before he entered the military. In chapter 1 there are details of his childhood and family. He was the second of four children in his Italian immigrant family, and by far the most unruly. As a child, he constantly dared to do things which most kids would avoid, either for fear of punishment, fear of getting hurt or killed, or from a simple lack of creativity to come up with the kind of ideas Louis thought about all day. He was a practical jokester and thief, among other things, and because he was small for his age, instead of fighting, he most often ran from his adversaries.
In chapter 2, his older brother Pete (Louis' exact opposite in kindness, behavior, respect for authority, and desire to succeed) decides to take Louis under his wing and hone his rebellion and propensity for running into something positive. He forces Louis to join the track team at school, then undertakes the task of personally training him into success.
As Louis begins to feel the attention and positive praise he was always seeking with bad behavior, chapter 3 outlines his very rapid ascent into running success. He makes up his mind that he will become a world class runner. After breaking several high school and collegiate records, Louis qualifies for the Olympic trials, and then makes the team.
Chapter 4 follows Louis to the 1936 Olympics in Germany, and Louis' first taste of the Nazi's (which wasn't altogether terrible). Though he does not medal in these Olympics, he does far better than anyone expected him to, given his age, inexperience, and short training. He leaves with his heart set on the 1940 games in Tokyo.
In Chapter 5, the conclusion to Part I of the book, Louis returns to finish college at USC. He continues to run and to train, but when he hears the news of the war, the United States' participation in it, and finally, the cancellation of the 1940 Olympics, he decides to join the Air Force. This is where Part II of the book picks up.