On the run from a killer, Annie and her mother flee to a remote farmhouse in New England. When Annie finds an unsent letter from a debutante in the Civil War, she is drawn to the story of a Southern girl immersed in a love that cannot see light in the American South. Determined to find out what happened to her, Annie risks bringing the killer to her farmhouse door. Underground chronicles two remarkable journeys - one across modern-day America and another through a country on the brink of its greatest historical change.