In the year 1861, a Union sentry sleeps at his post near a road on a high mountain cliff. Below him lies a narrow valley through which his military unit must pass in order to attack an unsuspecting Confederate army on the other side of the mountain. Suddenly, he awakens and sees an enemy soldier on horseback at the edge of the cliff observing the troop movement below. The sentry is struck by the beauty of the motionless figures silhouetted against the sky and is momentarily torn between his desire to preserve that beauty and to carry out his sworn duty to kill the enemy. He remembers the last words of his father, a wealthy Southern gentleman who, saddened by his son's decision to join the Union army, nonetheless admonishes him to "do his duty." The sentry lifts his rifle and fires, causing the horse and rider to plunge from the top of the cliff. Moments later, when the sentry is questioned by his sergeant, it is discovered that the Confederate soldier whom the sentry killed is none other than the sentry's own father.