The film begins with Solomon Northup, a slave, trying to make a pen and ink in order to send a letter somewhere. In flashback, we see Solomon living happily and freely as a violinist in Saratoga, New York, with a wife and two children. After his wife leaves to take a gig as a cook somewhere, Solomon meets two men who offer him a short term gig with a circus in Washington D.C.
Solomon travels to Washington with the men, who treat him to a lavish dinner. He thanks them for their generosity, but seems to overindulge. When he wakes up, he realizes he was drugged; he is in chains and is told that he is a slave by the name of "Platt." In spite of his protestations, Solomon is loaded onto a riverboat with other slaves, one of whom, Clemens, befriends him and tells him to keep quiet about his previous life and education.
After Clemens gets reunited with his master, Solomon is sold, along with a woman, Eliza, to a man named Ford. Eliza mourns getting separated from her two children, while Solomon attempts to ascend the hierarchy on the plantation using his wits. However, a white worker, Tibeats, resents Solomon's intellect, and attempts to kill him. Ford tries to save Solomon and sells him to another plantation, belonging to the more erratic and heavy-drinking slaveowner, Master Epps.
At Epps' plantation, Solomon knows to keep his head down and remain discreet about his education. There, he works hard, but has a contentious relationship with Epps. Epps has also taken an especial interest in Patsey, a slave girl, much to the chagrin of his ruthless and vengeful wife, Mistress Epps. Epps regularly rapes Patsey, and Mistress Epps abuses the girl in retaliation. At one point, Patsey begs Solomon to kill her and relieve her from her misery, but he refuses.
After getting sent to work on another plantation briefly, where he is able to make some money playing the violin, Solomon returns to Epps' plantation. There, he asks a white indentured servant, Armsby, who seems to be trustworthy, to deliver a letter for him. No sooner has he drafted the letter, however, than Armsby tells Epps. When Epps confronts Solomon about the letter, Solomon says that Armsby is lying to gain favor, and Epps believes him.
Not long after Patsey returns from a trip to a nearby plantation where she has received soap from the black mistress there. Epps believes that she has slept with Shaw and ties her to a nearby tree, before handing a whip to Solomon and ordering him to whip Patsey. Solomon does so for a short time, but then stops, at which point Epps takes over with a vengeance. Patsey is left with horrifying welts all over her back.
After hearing a white hired hand, Bass, speak out against slavery, Solomon takes a risk and tells him his story. He asks Bass to send a letter to his friends in Saratoga in hopes that he will be saved from the plantation. Bass agrees, and soon enough, Solomon is visited by a man from Saratoga who has come to take him home. Solomon returns to his family house, where his children are grown. He meets his grandson, who is named after him.