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...