The movie opens with a group of slaves receiving instruction on cutting sugar cane. A man sits languidly atop a wagon of the raw cane root, idly looking at the labor of the men. The scene shifts to a group of shacks. The slaves are eating. Solomon Northup notices the dark juice of blackberries and is inspired to create ink and a quill. Unfortunately, the scheme fails. The juice is too thin. Later, in the crowded slave quarters, a female stirs in her sleep and advances on Northup sexually, using his hand for release. Solomon flashes back to happier times with his wife and children and the title card flashes.
We see snippets of Solomon's life as a free man. He plays the violin exquisitely and is much in demand as a performer in his home of Saratoga. Later that night, after settling the children in their beds, he talks with his wife, who will shortly take the children with her as she works as a cook. He playfully expresses jealousy at being deprived of her cooking. The following morning he sees them off in a carriage. Later that afternoon, he encounters a friend who introduces him to two travelers (Brown and Hamilton) claiming to work with a circus-like show. They promise him an outrageous sum to accompany them to Washington DC and promise his return before his wife gets back. He agrees to join them.
We next see the trio at a restaurant in DC. His patrons plunk down a bag of coins in excess of their promised sum. They share wine and one of them watches closely as Solomon drains his glass. Everyone is having a great time. Suddenly, Solomon awakes in a dank cell, chained to the floor. In a series of flashbacks, we see his "friends" carry him up to his hotel room, excusing his drunken stupor to other patrons. Brown urges Hamilton that time is short, and they need to be done with it. Their departure ends the flashback, and we rejoin Solomon in the cell where he is told that he is a runaway Georgia slave. Despite his protests of being a free man, he has no papers. Solomon is mercilessly beaten and eventually sent into a slave pen with others.
He discusses his situation with Clemens, an apparently educated slave who advises him on the dire nature of their situation. Soon a mother (Eliza) and daughter are brought to the pen to join a previously captured son. She tries to keep a brave face while understanding the forthcoming tragedy. Under cover of darkness, they are pulled from their cell, chained and transported to a river boat. They are led to the hold, crammed with other human cargo. Clemens reiterates his advice that Solomon maintain a low profile, denying his ability to read and write. The encounter another slave (Robert) who wants to revolt and take over the ship. They contemplate their odds before opting for caution. Later that night, a slaver visits the hold and awakens Eliza to rape her. Robert attempts to stop the rape but is stabbed and killed. Clemens and Solomon are charged with dropping the body in the river, prompting Clemens to remark that Robert is better off in death. Later, they arrive at a dock. Clemens's master is waiting for them and immediately demands the return of his stolen property. Clemens gratefully scampers down into the embrace of his master, abandoning all evidence of his previously displayed intellect. Solomon has lost his only friend. After disembarking, a slaver (ironically named Freeman) calls his new property to their feet by announcing their names. He calls Solomon "Platt" which he obviously doesn't recognize. Solomon is slapped for denying the name.
The indignity of Freeman's slave operation is displayed as the naked slaves bath in buckets at his offices. Inside, he puts on his sales spiel to eager customers. A genteel plantation owner (Ford), expresses interest in Platt (Solomon) and Eliza. She begs him to take her children as well, but Freeman balks, swiftly selling her son to another buyer. Ford attempts to buy her daughter (clearly of mixed heritage), but again Freeman won't adjust his price. Ford can only afford to pay for the two. Eliza is distraught and wildly screams in her grief, upsetting the sale. Solomon is commanded to play the fiddle to lighten the mood. Ford transports his purchases back to his plantation. Eliza has been sobbing the whole trip. Ford's wife mentions that food and a night's rest will help her forget them. The next morning, the slaves are introduced to Tibeats, a slave handler, and Ford's overseer, Chapin. TIbeats sings a derisive song warning the slaves against escape as they perform their labor. They continue chopping timber and ultimately encounter a small band of native people with whom they share a brief respite. Solomon notices a stringed instrument and appears to remember his own violin.
The next day, Solomon (against Clemens advice) approaches Ford with a novel idea to transport the lumber via the river. Tibeats is extremely patronizing, but Ford is impressed with Solomon's opinions and is persuaded. The scheme succeeds, and Tibeats is embarrassed. Ford offers Solomon a violin to play as a reward. Back at the slave quarters Eliza is moaning in sadness over the loss of her children. Solomon is frustrated by the noise and debates her about surviving under Ford's "decent" treatment. Eliza argues back that Ford must surely realize that Solomon is not a slave, yet does nothing to free him. Solomon is given pause. Eliza is eventually sold off as Ford's wife can't "bear the noise." Over the next few days, Tibeats attempts to wreak petty vengeance on Solomon, leading to a verbal confrontation. Tibeats attempts to beat Solomon who fights back and gets the better of him. Chapin comes upon the scene and sends Tibeats scurrying. He warns him that he cannot protect him if he runs and implies that he will get Ford to straighten it out. Later, we find that Tibeats has gathered some thugs to lynch Solomon for daring to fight him. They have the noose around his neck and are preparing to hang him when Chapin returns, guns drawn. He chases them off, but allows Solomon to hang there, barely able to support his weight, on tip toes (punishment for striking a white man). Gradually, slaves emerge from their cabins and appear to take no notice of his plight. A woman furtively brings him some water, but quickly retreats. What appears to be hours later, Ford finally returns and slices the rope, saving Solomon. He drags him into the house for protection, but decides that he must be sold. Tibeats will not be denied his vengeance. It is here that we learn Ford has sold Solomon to a notorious plantation owner named Epps, who is known for merciless beatings. Epps reads a Bible passage, slanting the scripture to reinforce his ownership of the slaves. The next day brings a day of picking cotton. At the end of the day, the weight of each worker's bundles are marked. Solomon's yield is less than average. Slaves that picked less than the previous day receive lashes. Meanwhile, Patsey exceeds the best production of any worker by nearly double. Epps lingers around Patsey and offers extensive praise for her. It is clear he is fascinated by her, and his wife is none too pleased. Epps enters the slave quarters and rouses them from sleep, creating an impromptu dance where Patsey is the center of attention. Epps wife throws a heavy crystal decanter at Patsey's face, brutally scarring her. She demands that Epps sell Patsey, but he claims that he would send his wife away before losing Patsey.
Mistress Epps sends Solomon on an errand to the store. She hands him a list and notices him reading it. She makes it clear that he should not do it again. On his way to the store, Solomon is inspired to attempt escape, but stumbles into a lynching. Seeing the fate of the two men, his spirit is broken, and he continues to the store. Seeing the paper, he develops the idea to take a spare sheet each time so that he might craft a letter. Some time later, Epps dispatches Solomon to a nearby plantation owned by Shaw. Shaw has married one of his slaves and elevated her status (at least on his plantation). Patsey is there for a visit, enjoying the finery, but apparently Epps is jealous that Shaw might attempt to bed her. After a brief refreshment, Solomon convinces Patsey to join him. As they return to Epps' plantation, he is clearly drunk. Solomon whispers to Patsey that she avoid Epps, which Epps interprets as Solomon making a sexual advance. After a drunken chase around the yard, Mistress Epps intervenes, if only to express her disgust at her husband's obsession with Patsey. Later that night, Epps stumbles to the slave quarters and rapes her. Mistress Epps has had enough of her husband's affair and chastises Patsey before cruelly slashing her face. Later that night, Patsey begs Solomon to strangle her and dispose of her body. She can no longer bear the burden of Epps's rapes and his wife's torments. Solomon refuses, despite her entreaties. Some time later, we see that Epps's cotton crops have been devastated by insects. Two crops have been lost, and he decides to lend his slaves to a judge who can derive some use of them and pay the mortgage on their purchases. This returns us to the opening scene, where Solomon is cutting sugar cane. The judge notices Solomon's skill and recommends him to a neighbor seeking music for a party. As a bonus, the judge says Solomon may keep whatever wages he earns. The party is a fancy costumed affair, and Solomon clearly sees parallels between his old life as a free man and his forced servitude.
The party is over, and it's time to return to Epps's farm. As Solomon walks up to the house, Patsey's bloody eye indicates that her torments have continued. The cotton crop is in, and that means it's back to the fields. This time, they are joined by a white laborer (Armsby) who is picking to earn money to get back on his feet. Even though his yield is well below any other worker, he is spared the whipping that all the other slaves receive. In the quarters, he tends to Solomon's wounds and tells his story. He appears to be a decent man and a sympathetic ear. Solomon decides to risk trusting him with sending a letter North, in hopes of securing his freedom. He gives Armsby all his earnings from the party and swears him to secrecy. He will deliver the letter to him in two days. Solomon sets about creating ink and drafts the letter. Just that night, Epps enters the quarters and walks Solomon outside. Armsby has broken his word and told Epps everything. Fortunately, he told his story BEFORE Solomon gave him the letter. Thinking quickly, and playing on Epps's low opinion of slaves, he flips the story on Armsby, branding him a liar who is seeking to curry favor in order to get a job. This story persuades Epps and Solomon is spared. Later, Solomon burns the letter and watches as his hopes of freedom are extinguished in the ashes.
Some time later, we find a team of workers building a structure with a hired hand, Bass. Bass is from the North and holds strong views that fly in the face of Epps's strongly pro-slavery leanings. They chatter back and forth in front of Solomon, piquing his interest. Later, Epps is in a lather about Patsey. She's gone missing, and Epps thinks she has run away. He threatens all the women with violence over her loss; however, she has simply been back to Shaw's plantation to visit her friend. She attempts to convince Epps that she is faithful to him and went there to get some soap, a "luxury" that Epps's wife has denied her. She loudly asserts her worth and insists that she deserves to be clean. Epps is pushed over the edge by his wife's bickering and calls for Patsey to be tied to the whipping post. As he prepares to strike, he finds himself unable to inflict the punishment. Cravenly, he insists that Solomon do it. At first, Solomon attempts to be gentle, But Mistress Epps sees through the deception and nudges her husband to increase the severity. Epps points a gun to Solomon's head and claims he will kill every slave he sees if he doesn't whip Patsey harder. Faced with an unspeakable choice, he whips her harder, a pink mist of blood accompanying every new strike. After he pauses, Epps rushes forward, seething with rage and finally whips Patsey himself. The brutal punishment rends her flesh to shreds, and she collapses.
Solomon finds himself alone with Bass, the hired hand. He asks where he's from and when Bass replies, "Canada," Solomon offers convincing knowledge of the country. Bass asks how he's so well travelled, and Solomon explains his dire circumstance. Bass comes to believe Solomon's story and knows the horrible injustice of it. As they continue work, Solomon takes a chance and asks Bass to write letters to his friends in Saratoga. Bass agrees. Then the work is finished, and he leaves. A long shot lingers on Solomon. He has no idea whether Bass has kept his word. We don't know how long Bass has been gone, but the tears in Solomon's eyes indicate that perhaps he's beginning to think that he has been betrayed again.
Now we see a group of men tilling the soil and planting seeds. A carriage pulls up to the Epps plantation, and an official looking man calls out for Platt (Solomon). He answers and approaches the man (a sheriff). The man asks him some questions and motions to another man in the carriage. It is Mr. Parker, a shop owner from Saratoga and friend of Solomon's. With little further prompting, the sheriff is convinced, and Solomon rushes to embrace his friend. Epps is enraged and shouts empty threats. The sheriff rebuts the arguments and Parker helps Solomon into the carriage that will take him to safety. Patsey is there and calls out to him. Solomon leaps from the carriage to embrace her one last time before his departure. As he leaves, Patsey collapses in grief. Now Solomon has been carried home. Outside his door, he appears overcome at having been delivered from his nightmare. Upon entering, he sees his family. They are 12 years older, but overwhelmed at seeing him. His daughter has married and named their son, Solomon Northup. Tears flow as they gather around him and welcome him home. A series of title cards explain that he attempted to sue his kidnappers but failed. Northup became an abolitionist and aided many runaways in achieving their freedom.