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.