Soldier's Heart is a blunt depiction of Civil War combat. Based on the real-life experiences of Charley Goddard, the novel is an account of Charley's experiences from the time he enlists in the Union army at age fifteen to a couple of years after the Civil War when he is twenty-one and his life is a shambles. The title of the book is a phrase used by Americans to label Civil War veterans who seemed mentally lost because of the war. Paulsen points out that the same syndrome was called "shell shock" during World War I, "battle fatigue" during World War II, and is presently called "post-traumatic stress disorder."

From the moment Charley thinks, "I am not supposed to see this, God. No person is supposed to see this," Charley falls ever deeper into a depression in which he believes his own death is inevitable. Even comes to welcome it. He avoids other soldiers because they seem as doomed as he, and he does not want to become close to men who may die. The deaths of people he new from his Minnesota home, even the death of cocky Nelson, whom Charley knew for less than a day, weigh heavily on his mind. Soldier's Heart follows Charley's effort to withdraw into himself and away from the horror of the battles he fights.