Driving a carriage by the River Goose, Hiram has a vision of his mother, Rose, water dancing. The vision makes Hiram accidentally drive the carriage off the road, sending him and his half-brother and master Maynard into the water. Maynard can’t swim and calls out Hiram for help. Hiram is convinced he is about to die.
The narrative jumps back in time to when Hiram is a child. He explains that he has an extraordinary power of memory. The one person he cannot remember is Rose, who was sold. Now, Hiram walks down the Street (the part of Lockless plantation where the enslaved people live) to the cabin that belongs to Thena, a woman with a reputation for being mean. However, Thena takes him in, and a year and a half after Hiram moves in with her, she tells him about her husband, Big John, who died of a fever. After John’s death, their children were sold.
Hiram’s father is the master of Lockless, Howell Walker. Hiram is told that he is being brought to work in the house, and Thena warns him not to forget that the white people aren’t his family. Soon, Howell throws a party, during which Hiram is made to entertain the guests by showing off his supernaturally good memory. Maynard’s tutor Mr. Fields is especially impressed by Hiram’s skills, and soon after starts tutoring Hiram in reading and math. Meanwhile, falling tobacco prices mean that Virginia plantations like Lockless are in financial trouble, and more and more enslaved people are being sold into the Deep South. Howell tells Hiram that he is being appointed as Maynard’s personal manservant, which is supposedly an “honor.”
The narrative jumps forward seven years, to when Hiram is 19. It is the day before “the fateful race day” (when the book opens). Howell has arranged an engagement between Maynard and Corrine Quinn, the richest woman in Elm County. Early the next morning, Hiram wakes from a nightmare and notices that the lantern is on in Sophia’s room. Sophia is a young woman who is Howell’s brother Nathaniel’s enslaved “concubine.” Hiram has a brief conversation with her but is so nervous he can barely speak. Later that day, Hiram drives Maynard to the racetrack for race day. Almost all the white people in the local area are there, plus enslaved people and a few free black people. Hiram greets Corrine’s enslaved servant Hawkins.
Triumphant from his horse having won, Maynard makes Hiram take him to a brothel. Meanwhile, Hiram wanders into Freetown, the section of the town of Starfall where the free black people live. He goes to have dinner with Georgie Parks, his wife Amber, and their newborn baby, and gives the baby a wooden horse he carved as a gift. Hiram believes Georgie is connected with the Underground, and although he doesn’t really know what this is, he tells Georgie he wants to flee slavery and asks for help. Georgie denies that he is involved and tells Hiram to be happy with what he has. Driving home, Hiram experiences the vision of Rose and crashes the carriage into the River Goose. After being unconscious for three days, he wakes up in a sunny room. He drifts in and out of consciousness, and when he wakes again Howell is there, saying that Maynard is dead. Later, Hiram goes for a walk with Sophia, who tells him that he reminds her of the man she was with back when she lived in Carolina.
Soon after, Corrine requests a meeting with Hiram and says that she has heard about his genius and would like him to work for her. Hiram is worried about leaving Lockless but doesn’t have much choice. Christmas comes, and the enslaved people at Lockless as well as other local black people discuss Santi Bess, Hiram’s grandmother. According to legend, Santi Bess liberated herself and 48 other enslaved people by leading them back to Africa through the River Goose. Hiram dismisses the story as “preposterous.”
Hiram speaks to Georgie about fleeing again, and after some further reluctance, Georgie says to meet him in the same spot in one week. The next day, Hiram and Sophia agree to escape together. A week later, they go to meet Georgie, but are ambushed by Ryland’s Hounds (slavecatchers). They are taken to jail together, but within a day Sophia is taken, leaving Hiram sharing a cell with a boy of around 12 and an old man who is ruthlessly tortured and degraded by the guards. Eventually, Hiram is sold, and is brought to a dark pit, where he spends an unknown length of time. At night, he and other black men are brought out into a forest and forced to run for their freedom while white men chase after them. Hiram is always caught and put back in the pit, and the whole thing starts over again. He does this again and again, until one day, when he falls and is caught, it is not the usual white men standing over him, but Hawkins.
When Hiram wakes up, Corrine tells him that he was able to escape drowning in the Goose due to a special power called Conduction, and that it was this same power that allowed Santi Bess to lead all those people to freedom. She tells Hiram that he is free now, but that he is still going to “serve.” As Hiram gets better and is able to get up and eat, he realizes that he is at Corrine’s property, Bryceton. Corrine is secretly an abolitionist, and Bryceton is a station on the Underground Railroad. Hiram is trained to be an agent, engaging in both taxing physical training and by resuming his studies with Mr. Fields, who is also an agent. He learns to start imitating documents, including letters, authorizations, and bills of sale. Meanwhile, he also tries to gain mastery over Conduction—the ability to move through space using the power of memory—yet struggles to do so. Mr. Fields reveals that his real name is Micajah Bland.
After four months, Hiram leaves for Philadelphia. He is overwhelmed by the sights, smells, and sounds of the city, and particularly by seeing so many free black people, some of whom are even wealthy. He meets Otha and Raymond White, and starts living with Otha in the city. Soon after arriving, he eats a piece of gingerbread that induces a memory from his childhood and prompts Hiram to briefly experience Conduction.
Hiram takes a woodworking job for three days a week, spending another three days on work for the Underground. He meets Raymond and Otha’s large extended family and learns about Lydia, Otha’s wife, who remains enslaved in Alabama. One day, Hiram is walking through the city when he is kidnapped by illegal slavecatchers and taken to the forest. There, he is rescued by Bland, who kills the slavecatchers. The next day, Raymond admits that he knows that Sophia is back at Lockless.
Bland is going to Alabama to help Lydia and her two children escape, and Hiram forges documents in support of the mission. Meanwhile, Raymond, Otha, and Hiram set off for a big abolitionist convention in upstate New York. Hiram is fascinated by what he sees and hears at the convention. A woman approaches him and introduces herself as Kessiah, explaining that she is Thena’s daughter. She is free now, living in Philadelphia, and also connected with the Underground. The next day, Hiram talks to Moses, a legendary figure in the Underground. Moses dismisses the myths about her powers and tells Hiram to call her by her real name, Harriet. She asks him to join her on a mission to Maryland, and he agrees. News arrives that Bland’s mission failed at the final hour; he is dead, and Lydia and the children have been re-enslaved. Otha is devastated.
The group returns to Philadelphia, and shortly after Hiram sets off on his journey to Maryland with Harriet. They travel there via Conduction, which Harriet explains is “just like dancing.” In order to conduct them, Harriet tells stories about her childhood, including the boy, Abe, who first inspired her to fight for freedom. After arriving in Maryland, Hiram meets Harriet’s siblings, who they will be taking to freedom in Philadelphia. He intervenes in an argument between Harriet’s brother Robert and his partner, Mary, who doesn’t want him to leave. After, Robert confesses to Hiram that Mary is pregnant with their white enslaver’s child, which he struggles to cope with. Harriet successfully conducts her siblings back to Philadelphia.
Raymond gives Hiram a letter summoning him back to the Virginia Underground. Before leaving Philadelphia, he promises Kessiah that he will bring Thena to her. After reuniting with Corrine, Hiram asks that she promise to help him take Thena and Sophia to freedom, but she is reluctant, and this doesn’t cohere with her own plans.
When Hiram returns to Lockless, Howell embraces him with heartfelt emotion. The property has fallen into a state of ruin, and Hiram doesn’t recognize any of the few enslaved people left there. He reunites with Thena and Sophia, who now has a baby called Caroline. Carrie is Nathaniel’s daughter. Hiram works in the big house, acting as a personal servant and companion to Howell, who wants Hiram to manage the plantation after he dies. Hiram looks through the ledgers and learns that Lockless is deeply in debt. Meanwhile, Hiram, Thena, Sophia, and Caroline form a kind of family “unit.” During a conversation with Sophia, Hiram suddenly feels like he fully understands Conduction for the first time. He tests it out and realizes that he’s mastered it at last.
One day, Hiram and Sophia find that Thena has been beaten, her cabin destroyed, and the money she had been saving to buy her freedom stolen. They all start sleeping in Thena’s cabin together in order to protect her. At Christmas, Corrine comes to Lockless, and Hiram asks her again to help him get Sophia and Thena out. Corrine realizes that she has bought Sophia from Nathaniel and that she is therefore safe, but refuses to help him get them North. Hiram resolves to get them out anyway by working alone. This means he will have to tell each of them about the Underground, Philadelphia, and Conduction, which he does, starting with Sophia. He then tells Thena and explains that he is going to organize a reunion between her and Kessiah. Thena reacts with horror and fury.
Realizing that he needs an object from very deep in his memory to conduct more than a short distance, Hiram retrieves a shell necklace from Howell’s desk that Rose gave him the day she was sold. This triggers a memory that he had suppressed: Rose tried to run away with Hiram, and was on the run for three days before they were caught. Howell sold her as punishment. Thena explains to Hiram that her angry outburst was caused by her enormous pain. Soon after, Hiram conducts Thena to Philadelphia, where she is met by Kessiah and Harriet. Hiram immediately returns to Lockless.
That fall, Howell dies. Before his death, he arranged for Corrine to buy Lockless and take over its debts. Corrine and Hiram turn the plantation into an Underground station, which Hiram manages. He, Sophia, and Caroline live there together as a family.
