John Brown and his "Pottawatomie Rifles", including three of his sons, in territorial Kansas meet a young runaway slave. The slave, using the nickname Onion, joins John's group; John believes Onion is a girl and gives her a new store-bought dress, intended for one of his daughters. Throughout the series whites believe Onion is a girl, but the blacks see that he's a boy. Brown's goal is to free all the slaves, which involves killing slavers, in a massacre. John's son Frederick ends up being killed, while another son abandons the group, tired of fighting.
