Billy Forest and Harley Cobb grow up as neighbors and best friends on adjacent farms in 1850's Ohio. When the Civil War starts, Billy is apprenticing with a law firm in Columbus, looking forward to life as a trial attorney. Harley is working his family farm with his father and brothers.
Billy and Harley join the 23rd Ohio Volunteers in 1861 and spend the next four years immersed in the horror, tedium and depredations of war. When they muster out in July 1865 they find that they are both changed in ways they don't understand. Besides each other, the only thing they can rely on is their guns. On the way home, they get into a gun battle in which a saddle partner is killed and Billy is seriously wounded. Billy blames himself for their friend's death because he couldn't get his Colt .44 out quickly enough to do his part. When he recovers he swears he will never let another friend down like that again. He becomes obsessed practicing with his guns.
When they get home, Billy and Harley find out they don't fit in with their families anymore. They are experiencing a condition known then as "soldier's heart," now as PTSD. After Billy is forced into another shoot-out with railroad men who try to confiscate his family's farm, he decides he must go west to escape further trouble and to figure out what is wrong with him.
Billy and Harley set out on a trail that ultimately takes them to the Colorado Territory. Along the way they winter in St. Louis. There, they meet Joab Baker, who befriends them and becomes Billy's mentor in the ways of the west. Billy learns that conflict in that untamed land can quickly become deadly, and hesitation fatal. Baker teaches him how to pull his gun out fast and shoot straight.
Billy and Harley also meet Gabe in St. Louis. Gabe is a former slave who fought with the famous 54th Massachusetts Regiment in the war. Billy and Harley inspire Gabe with their enthusiasm about the freedom they expect to experience in the west, and when spring comes, Gabe decides his future is on the trail with his two new friends. The three of them quickly become inseparable; and they are a force to be reckoned with in a fight.
They join a wagon train in western Missouri, and work as outriders and guards on their way across Kansas on the Santa Fe Trail, and then into southeastern Colorado Territory. Along the way, Billy tries his first case when he volunteers to defend one of the cooks who is wrongfully charged with stealing a bottle of whiskey from a small store.
The three friends battle Cheyenne on the plains and bushwhackers attempting robbery and rape in southeastern Colorado. They then ride towards the gold fields up north and find Joab Baker fighting a rough bunch of claim jumpers at Mayhem Gulch, near Golden City. After helping Baker out of a shooting scrape they head to Boulder City. The natural beauty and abundance of opportunities in the area convince them to try to make their lives there.
Billy finds a new mentor in Boulder. He befriends an elderly Chinese laundryman who begins to teach him the practice of "chan," or Chinese zen. Lin-Chi helps Billy deal with the symptoms of his soldier's heart. The flashback images from the war begin to dissipate and Billy learns the virtue of living his life in the present. His new, enhanced awareness becomes invaluable in helping him manage the conflict that has become central to his life, whether that be in the form of hostile witnesses in court or guns in the street.
Billy's new approach to life serves him well during hard-fought litigation that pits him against a railroad man and two top attorneys from a big-city firm. However, his ultimate test comes just as he is celebrating the end of that trial. He is forced into a gunfight in which he must kill, or be killed by, his good friend Joab Baker. He relies on the training he received from both of his mentors to do what he must.