It's the early years of the Civil War. Mary Peake, a free biracial woman, believes illiteracy is like a closed door and devotes herself to educating the children of slaves. Under a large oak tree in a secluded field, Mary prepares the children for a future when they might be free. Brutal slave patrols are in constant pursuit, determined to capture and punish her for violating Virginia law. Meanwhile, Benjamin Franklin Butler, a discredited Union general, is banished to command nearby Fortress Monroe. When escaped slaves seek sanctuary within the fort, Butler must decide whether to defy Federal law, which requires him to return the men to their Rebel owners. Mary forms a turbulent bond with the General and−with growing confidence−she pursues the broader goal of ending slavery.