The narrator, Sarny, is a captivating heroine. At twelve years old, she stands on the brink of womanhood. She instantly garners the reader's sympathy due to her unfortunate circumstances and earns admiration for her courageous persistence. Unjustly rumored to be a witch because of a "stuck tongue" that makes her seem peculiar, Sarny is also parentless; her "birthing mammy" was sold away, and Clay Waller might be her father. She quietly defies her status as a slave by nurturing rebellious thoughts against her masters. Though she is a reserved girl, her mind is always active. With a sharp awareness of the harsh world that confines her, she discovers that learning to read and write is the best way to contribute to her own freedom and her people's liberation. Eagerly, she seeks out the knowledge that Nightjohn offers.

Alice and Mammy illustrate the peril of Sarny's quest for knowledge. Alice, a few years older than Sarny, is also considered peculiar by both whites and blacks. Like Sarny, Alice is headstrong and finds herself in an uneven struggle against the Waller family after a single mistake leads to her punishment. Bravely but recklessly, she attempts to escape, only to be brutally attacked by the slave master's hunting dogs. Her fate foreshadows what Sarny might face in the coming years.

Mammy, who has raised Sarny as her own daughter, strives to protect her. Initially, she warns Sarny against becoming literate, aware of the severe punishment that awaits any slave caught reading or writing. When Sarny inadvertently reveals to Waller that she can recognize a word, Mammy takes the blame and endures the humiliation of being harnessed like a draught horse. Her punishment ceases only when Nightjohn steps forward to accept responsibility.

Sarny quickly recognizes Nightjohn as a valuable mentor. She admires his stoic resilience during the humiliating torture intended to break him. She appreciates the bold way he offers to exchange his knowledge of the alphabet for the tobacco leaf she has saved. She is drawn to the sound of his voice ("thunder long way off, moving in a summer sky") as he teaches. She respects how he stands firm against Mammy's advice to keep his knowledge to himself: what slavery does to blacks "has to be written," he asserts. After he is savagely mutilated for aiding Sarny and later escapes from the Waller plantation, she joins Nightjohn in running a clandestine "pit school" at night for slaves. Nightjohn becomes the haunting yet living symbol of the determination to learn, resist, and gain freedom.

The book's dedication to "the memory of Sally Hemmings, who was owned, raised, and subsequently used by Thomas Jefferson without benefit of ever drawing a single free breath," underscores the theme of persistent black resistance to slavery. It also highlights the historical irony that slavery persisted in the United States almost a century after the writing of the Declaration of Independence, the Constitution, and the Bill of Rights.

However, this straightforward dedication is somewhat misleading. Paulsen had written an essay about Sally Hemmings intended to precede the novel. Delacorte Press argued that including such an essay was inappropriate for a novel aimed at adolescent readers. Paulsen remains adamant that the next edition of the novel will feature the original preface.