The novel The Underground Railroad opens with the story of Ajarry, a young woman who is captured by slave traders on the African continent and sold in America. Separated from her family and reduced to her value on the auction block, Ajarry ends up in the southern state of Georgia on the Randall tobacco plantation. She takes several husbands and gives birth to a handful of children, but none survive except one named Mabel. Ajarry’s one small act of liberation on Randall is to begin a vegetable plot on a small piece of land between slave cabins that her daughter Mabel and eventually her granddaughter Cora will inherit. Ajarry dies as a slave of an aneurysm while picking cotton.
The plot picks up with Cora, Ajarry’s granddaughter. When Cora is still a young girl, Mabel escapes Randall without her daughter, who is relegated to Hob, the cabin for slave outcasts. Alone on the plantation, Cora proves her independence and determination by protecting Ajarry’s legacy, the vegetable plot, from the intrusions of another slave, Blake, and his dog. She becomes a stray, growing up to suffer sexual assault and social derision at the hands of other slaves.
When Cora is a teenager, another slave, Caesar, approaches Cora with a plan to escape. She declines, thinking it is a joke or a trap. The slaves celebrate Old Jockey’s birthday and Cora tries to protect a boy named Chester from a beating at the hands of their master, Terrance Randall. Both Cora and Chester are brutally whipped as punishment. After Cora recovers from her wounds, she makes up her mind to escape with Caesar.
One night, Cora and Caesar set out to cross the swamp that surrounds the plantation. Lovey, Cora’s friend, suspects they plan to escape and insists on going with them. Shortly after, a group of pig hunters finds them in the woods. In the altercation, Cora kills one of them, a teenage boy, in self-defense. Lovey is captured and taken back to Randall, but Cora and Caesar manage to escape to the home of a white abolitionist named Mr. Fletcher, who sends them on their first journey on the Underground Railroad.
Arriving in South Carolina, the station agent, Sam, welcomes them, and provides them with the alibis of Bessie Carpenter and Christian Markson. They learn that the government in South Carolina has begun buying up slaves and setting them up with jobs for a wage, rooms in dormitories, and health care, essentially allowing them to live as free people. Under this scheme, Cora learns to read and write as she works first as a nanny for a family called the Andersons and then as a living model in an exhibit on American history in a government-funded museum. Caesar enjoys his work in a factory. The two attend social dances and begin to settle in, putting off the next leg of their journey as they adjust to freedom. Sam overhears the sinister truth of South Carolina, however, and shares it with the pair: the government program includes forced sterilization of black women and a syphilis study of black men without their consent. Before Sam, Cora, and Caesar can decide what to do with this news, Ridgeway, a feared slave catcher, finds them in South Carolina. Caesar is captured and lynched by a mob. Sam’s house is burned to the ground. Cora, however, escapes to the Underground Railroad once more.
The novel interrupts the saga of Cora’s escape to introduce Arnold Ridgeway, a feared slave catcher. Ridgeway has built up a reputation for being the most effective slave catcher in the South, often traveling to the far northern states to recapture runaway slaves. He was hired by the older Randall to find Mabel when she ran away years ago, but failed. When Terrance Randall enlists him to find Cora, and he learns she is Mabel’s daughter, he becomes obsessed with finding her and eradicating the Underground Railroad from the South.
In the next chapter, Cora travels on the Underground Railroad to North Carolina. There, she hides in the attic of a white couple named Martin and Ethel. North Carolina has recently established a genocidal society. In reaction to slave rebellions, the white population sold all of its slaves to other southern states and is in the process of lynching any last black people they find within state borders. Cora watches these lynchings in the park across the street from her attic window. Cora falls ill after months of hiding in the attic. While Ethel is nursing her back to health, slave catchers raid the house. The townspeople lynch Martin and Ethel and Ridgeway recaptures Cora.
A brief chapter tells the story of Ethel’s life. She grew up wanting to save “savages” in Africa as a missionary, but became a schoolteacher instead. The text alludes to Ethel’s suppressed lesbian identity throughout her life, but Ethel marries Martin and they have a daughter. They move from Virginia to North Carolina, where Martin is from, when his father dies. Martin inherits the abolitionist cause from his father and becomes involved with the Underground Railroad. Ethel is a nervous, reluctant bystander until her chance to play missionary when Cora falls ill.
The next chapter returns to Cora’s plight. Ridgeway and his assistants, Bozeman and Homer, travel with Cora and another runaway, Jasper, through Tennessee. Ridgeway shoots and kills Jasper in cold blood to stop him from singing hymns. Tennessee is a barren, fire-ravaged landscape, and the few white settlements they come across are quarantined with yellow fever. Their journey is interrupted when a group of free black Underground Railroad agents rescue Cora from Ridgeway.
Cora and the Underground Railroad agents, including one named Royal, travel on the Railroad to a farm in Indiana owned by John Valentine, a free black man. The farm is home to numerous freed black people and escapees. Everyone labors equally and lives in community. Cora shares a cabin with Sybil and her daughter Molly, whom she becomes friends with. She also embarks on a tentative romantic relationship with Royal. There is disagreement on the farm about the best way to advance black progress: gradually or inclusively. Meanwhile, the white settlers who neighbor the Valentine farm grow angry at the free black community. They form a mob and ransack the farm, killing many members of the community, including Royal. Ridgeway returns with them and recaptures Cora.
In an interlude before the last chapter of the novel, readers finally learn the fate of Mabel, Cora’s mother. Cora resents her mother for leaving her behind on the plantation and escaping to freedom. As it turns out, Mabel made it no farther than the swamp surrounding Randall before turning back to reunite with her daughter. A poisonous cottonmouth snake bit her in the reeds, however, and she died in the swamp.
In the final chapter, Ridgeway forces Cora to show him the Underground Railroad station near the ransacked Valentine farm in Indiana. Grieving the loss of Royal and her community, Cora leads Ridgeway and Homer to the station. About to descend into the station, Cora ambushes Ridgeway and pushes him down the cellar stairs. Leaving him mortally wounded on the platform, she boards the train car for the last time in the novel. This time she pushes the lever herself. A few days later, Cora emerges from the Railroad. She comes across a caravan of traveling wagons. She ignores the white passersby but is greeted by a black man named Ollie in one wagon. He offers her a ride heading toward St. Louis and then on to California. Cora accepts and joins the caravan.