Henry Shackleford, a young slave living in the Kansas territory in 1856, has a run-in with abolitionist John Brown, who mistakes him for a girl and promptly "frees" him. Believing Henry to be his good luck charm, Brown welcomes him into his ragtag army and nicknames him "Onion." Donning a dress for the first time while dodging bullets, Onion quickly awakens to the life or death stakes in the battle over human bondage and witnesses Brown's divinely inspired violent agenda: to free the land of slavery by any means necessary.

Separated from Brown's army, Onion and fellow slave Bob leave the fight in Kansas only to cross paths with pro-slavery "red shirts" who escort them to the slave-trading town of Pikesville, Missouri. Onion is taken under the wing of Pie, a mixed-race prostitute working out of the local hotel, who instructs him in the ways of the world despite her discovering his dangerous secret. Onion soon finds himself enmeshed in a mounting slave insurrection where he learns the consequences of his own actions.

Following the battle in Pikesville, John Brown, now one of the most wanted men in America, travels north with Onion to raise funds for the abolitionist cause. They take refuge at the home of famed orator and "King of the Negroes" Frederick Douglass and his two wives. When Brown and Douglass clash over methodology, Onion discovers just how complex the fight for justice and human equality really is.