Consisting of 43 chapters, the novel begins with Huck Finn introducing himself as someone readers might have heard of in the past. Readers learn that the practical Huck has become rich from his last adventure withTom Sawyer (The Adventures of Tom Sawyer) and that the Widow Douglas and her sister, Miss Watson, have taken Huck into their home in order to try and teach him religion and proper manners. Instead of obeying his guardians, however, Huck sneaks out of the house at night to join Tom Sawyer's gang and pretend that they are robbers and pirates.
One day Huck discovers that his father, Pap Finn, has returned to town. Because Pap has a history of violence and drunkenness, Huck is worried about Pap's intentions, especially toward his invested money. When Pap confronts Huck and warns him to quit school and stop trying to better himself, Huck continues to attend school just to spite Pap. Huck's fears are soon realized when Pap kidnaps him and takes him across the Mississippi River to a small cabin on the Illinois shore.
Although Huck becomes somewhat comfortable with his life free from religion and school, Pap's beatings become too severe, and Huck fakes his own murder and escapes down the Mississippi. Huck lands a few miles down at Jackson's Island, and there he stumbles across Miss Watson's slave, Jim, who has run away for fear he will be sold down the river.
Huck and Jim soon learn that men are coming to search Jackson's Island, and the two fugitives escape down the river on a raft. Jim's plan is to reach the Illinois town of Cairo, and from there, he can take the Ohio River up to the free states. The plan troubles Huck and his conscience. However, Huck continues to stay with Jim as they travel, despite his belief that he is breaking all of society and religion's tenets. Huck's struggle with the concept of slavery and Jim's freedom continues throughout the novel.
Huck and Jim encounter several characters during their flight, including a band of robbers aboard a wrecked steamboat and two Southern "genteel" families who are involved in a bloody feud. The only time that Huck and Jim feel that they are truly free is when they are aboard the raft. This freedom and tranquility are shattered by the arrival of the duke and the king, who commandeer the raft and force Huck and Jim to stop at various river towns in order to perform confidence scams on the inhabitants. The scams are harmless until the duke and the king pose as English brothers and plot to steal a family's entire inheritance. Before the duke and the king can complete their plan, the real brothers arrive. In the subsequent confusion, Huck and Jim escape and are soon joined by the duke and the king.
Disappointed at their lack of income, the duke and the king betray Huck and Jim, and sell Jim back into slavery. When Huck goes to find Jim, he discovers that Jim is being held captive on Silas and Sally Phelps' farm. The Phelps think Huck is their visiting nephew, Tom Sawyer, and Huck easily falls into the role of Tom. Tom Sawyer soon arrives and, after Huck explains Jim's captivity, Tom takes on the guise of his own brother, Sid. After dismissing Huck's practical method of escape, Tom suggests they concoct an elaborate plan to free Jim. Tom's plan is haphazardly based on several of the prison and adventure novels he has read, and the simple act of freeing Jim becomes a complicated farce with rope ladders, snakes, and mysterious messages.
When the escape finally takes place, a pursuing farmer shoots Tom in the calf. Because Jim will not leave the injured Tom, Jim is again recaptured and taken back to the Phelps farm. At the farm, Tom reveals the entire scheme to Aunt Sally and Uncle Silas. Readers learn that Miss Watson has passed away and freed Jim in her will, and Tom has been aware of Jim's freedom the entire time. At the end of the novel, Jim is finally set free and Huck ponders his next adventure away from civilization.