Susan Chilmark is a beautiful fourteen-year-old girl during the Civil War who decides to do something to support the South (Confederacy). She and her best friend, Connie, collect silk dresses from all the ladies of Richmond, Virginia to make a balloon that will be used to spy on the Yankees (the North). Susan's mother is a half-crazed woman who goes into screaming tantrums and calls her a "Yankee brat". Susan meets her handsome brother, Lucien, banished from the family, discovers he's an abolitionist, a person who hates slavery and works to end it. Susan also discovers that both of her parents have secrets. Her father slept with a black servant, and her mother had an affair with a Yankee. Susan grows up in the two years of the book and learns to follow her father's advice to "do what you think is right. Even if it hurts those around you. Those you love." Susan gives her balloon to the North when she understands this advice, becoming a traitor to the South. She falls in love and marries Tim, a Yankee correspondent.
Susan learns in the end that slavery is wrong because a human has the right to be treated decently. She knows that slavery is evil, and she gives her balloon away to the North because she has to help end slavery.
The conflict are North vs. South, Susan vs. her family and her inner struggle to do what's right.