Jim is Miss Watson's slave, who runs away after overhearing her saying she plans to sell him to a trader down in New Orleans.
He hides on Jackson's Island until Huckleberry Finn finds him; though propelled by differing motives, they make their way down the river on a raft for more than a thousand miles.
Jim is eventually betrayed by the Duke and King for forty dollars' share of a fictitious two hundred dollar reward promised by a forged handbill that had been carried on the raft to provide an excuse for carrying a likely runaway slave. Not long after, the Duke and King are run out of town on a rail after Jim exposes their Royal Nonesuch plot.
Jim is kept captive at Mr. Phelps' home, awaiting a response from the fictional plantation from which he is alleged to have run; Huck and Tom Sawyer construct an overly-elaborate plot to free him but Jim abandons the plot to assist Tom who is shot in the escape. For his assistance, Jim avoids a likely hanging - when Tom returns to lucidity, he explains that Miss Watson had passed away and freed Jim in her will.