A young slave living in the Kansas territory in 1856 has a fateful run-in with abolitionist John Brown, who frees him and nicknames him Onion.

Onion and fellow slave Bob run into pro-slavery red shirts who take them to the slave-trading town of Pikesville, Mo.

John Brown and Onion travel north, taking refuge at the home of Frederick Douglass and his two wives; Brown and Douglass clash over methodology.

John and Onion continue their fundraising mission, heading to Canada on foot; the legendary Harriet Tubman endorses Brown in a room full of new recruits.

Onion and John Cook head to Harper's Ferry to prepare for Brown's arrival.

The plan to take control of Harper's Ferry is finally put into action; however, a crucial mistake threatens the success of the entire operation. With shots firing, Onion is sent out to get reinforcements.