David Wilson has earned the unfortunate nickname "Pudd'nhead" from his fellow townspeople, who fail to understand his combination of wisdom and eccentricity. However, he is eventually able to redeem himself by simultaneously solving a murder mystery and a case of transposed identities.

Two children, a white boy and a mulatto, are born on the same day. Roxy, mother of the mulatto and a slave, is given charge of the children; in fear that her son will be sold, she switches the babies.

The mulatto, though he grows up as a white boy, turns out to be a scoundrel. He sells his mother and murders and robs his uncle. He accuses Luigi, one of a pair of twins, of the murder. Pudd'nhead, a lawyer, undertakes Luigi's defense. On the basis of fingerprint evidence, he exposes the real murderer, and the white boy takes his rightful place.

This classic book, full of grim humor and Twain's trademark style,implicitly condemns a society that allows slavery. It concludes with a series of witty aphorisms from Pudd'nhead's calendar.