Dawson's Landing, Missouri, is in 1830 a bucolic little town on the Mississippi River, complete with white picket fences, perfectly groomed yards, and a cat asleep in the front window of every house. Behind the town stretches the countryside, the town's means of economic support; there, slaves raise pigs and grow grain. Although the rate of economic growth has slowed, Dawson's Landing is still a thriving, if sleepy, town. York Leicester Driscoll (Judge Driscoll) is its chief citizen, a judge and a member of an old Virginia family, a heritage of which he is very proud. He lives with his wife and his sister, Mrs. Rachel Pratt. Neither the judge and his wife nor Mrs. Pratt have any children; this is the chief sorrow of their otherwise happy lives.
Pembroke Howard, a lawyer and another Virginian, is another leading citizen and Judge Driscoll's closest friend. Percy Northumberland Driscoll is the judge's brother. He is a wealthy speculator who has seen all of his children die from various ailments which the town's old-fashioned doctors were not able to treat. On February 1, 1830, both Percy Driscoll's wife and one of his slaves, a woman named Roxana, give birth to baby boys. Mrs. Driscoll dies a few days afterwards, and Roxana, or "Roxy," is left to raise both babies.
Also in February of 1830, a young man named David Wilson comes to Dawson's Landing. He is a lawyer from upstate New York who has come to seek his fortune. On his first day in the town, he makes a clever, sarcastic comment about owning half a dog that goes over the heads of the townspeople, who think he's an idiot and not in his right mind. The mistake dooms his law practice and earns him the nickname "Pudd'nhead," a name that sticks for the next twenty years. Since he has no clients, Pudd'nhead busies himself with small surveying and accounting jobs. He also engages in experiments of a more or less scientific nature. One of these is fingerprinting: Pudd'nhead Wilson collects the fingerprints of nearly everyone in town. One day he is in his study when he overhears Roxy flirting with another slave outside his window. She dismisses the man because she considers him "too black." Pudd'nhead looks out the window and notes that, although her speech marks her as black, Roxy appears to be white. She is only one-sixteenth black, but she is nevertheless a slave. Roxy has both her infant son, whom she has named Valet de Chambre after a phrase she overheard, and Percy Driscoll's son, who has been named Thomas a Becket Driscoll, with her when she stops outside Pudd'nhead's house. Chambers and Tom, as the infants are called, are identical in appearance. They are only distinguishable from one another by their clothing: Chambers wears rough garments, while Tom wears a fine gown and a coral necklace. Pudd'nhead goes outside to talk to Roxy and takes the fingerprints of both boys. Roxy tells him that she is the only one who can tell the two infants apart when they are not dressed.
Two months later, Pudd'nhead again takes sets of fingerprints from the two babies. The day after Pudd'nhead takes the prints, Percy Driscoll finds that some money has been stolen from him. This has happened several times recently, and he erupts in anger. Summoning his slaves, he demands that they identify the guilty party. That individual, he says, will be sold "down the river" to one of the Southern plantations, where they will suffer much more than they have under Driscoll. Roxy is glad that she has recently undergone a religious conversion at a local revival, and that it was not she who stole the money, although she acknowledges to herself that under normal circumstances she would have taken it. The narrator digresses for a moment to note that Roxy's way of thinking is average for a slave: anyone subject to such treatment, he implies, has the right to steal from their enslaver. All three of Driscoll's other slaves confess to the theft and beg for mercy. He tells them that they will be sold locally, and they are quite relieved. Going to bed that night, Percy Driscoll congratulates himself on his magnanimity and records the event in his diary for his infant son to read in the future.
Each of these chapters begins, like all the other chapters in the book, with a maxim from "Pudd'nhead Wilson's Calendar," a work that seems to be similar to Benjamin Franklin's "Poor Richard's Almanac." Wilson's Calendar, though, is filled with wry, pessimistic sayings of the kind that originally got him in trouble with the townspeople of Dawson's Landing. These sayings are obscure and often irrelevant to the plot of the novel. While they contain truisms of the kind that Twain himself became famous for writing, they do not hold the key to interpreting this novel. Instead, they exist as a kind of parallel text, which serves to remind the reader that other ways of thinking are available besides those of Dawson's Landing. These abstractions, however, may not be able to describe the world at hand, particularly its horrors.
Pudd'nhead Wilson is similar to Benjamin Franklin in his penchant for experimentation, too. His projects are dismissed by the townspeople as part of his general eccentricity, but in reality they represent efforts to develop an alternative classification system that may be more thorough and more egalitarian than the legally- and racially-based one that labels the white-looking Roxy as black and differentiates the status of the otherwise identical babies Tom and Chambers on the basis of the race of one of their great- great-great-grandparents.