In 2007, Dr. Martin Keller cut into a dying teenager's chest in the emergency room to give him a chance at life and changed the treatment of gun injuries at St. Louis Children's Hospital dramatically. Keller arrived as director of trauma when the hospital was beginning to see more gunshot wounds than any children's hospital in the country. Amid the chaos, he and other caregivers were tasked with making improvements to create a smoother running trauma operation. Under the Gun looks inside the emergency and operating rooms as physicians, nurses and surgeons tackle the overload and social workers strive to limit repeat gun injuries. It also examines the lives of victims and families, including a 6-year-old girl critically injured with a shotgun by her brother, a 12-year-old boy shot in the head and killed by a friend with a gun found on a bed, and a high school senior shot in the back while walking to school.