Rumors of an imminent duel between Judge Driscoll and Luigi continue to swirl around Dawson's Landing. Wilson carries Luigi's challenge to the Judge; however, Driscoll refuses to fight an assassin on the field of honor. Should the Judge run into him elsewhere, he will be ready. Thus, Wilson conveys to the twin that he must be prepared to kill the Judge upon seeing him, or else risk death himself.
Tom has returned from St. Louis and sneaks into his uncle's house, prepared to carry out the planned robbery. He disguises himself in women's clothing and blackens his face with cork. Additionally, he carries along the Indian knife for protection. Tom's initial plan is to sneak into his uncle's bedroom and steal the key to the safe. However, he finds the Judge downstairs, asleep on the sofa. The old man's tin cashbox stands nearby, closed. Close to this is a pile of bank notes. As Tom seizes his prize, the Judge grabs him and starts calling for help. Tom plunges the knife into his uncle, and he is free. He flings the knife onto the ground and runs upstairs to his room, just as the twins are entering the house and standing over the body. By the time Tom escapes out the back, Mrs. Pratt, the servants, and various neighbors have joined the twins over the corpse. As Tom passes through the gate, three women rush by him.
Tom is able to escape Dawson's Landing and travels to St. Louis. He learns in the following day's newspaper that Luigi has been arrested for his uncle's murder. The twin's grudge against the Judge, combined with his presence at the scene of the crime and his ownership of the murder weapon, have created a strong circumstantial case against him. Luigi is indicted for murder, while Angelo is indicted as an accessory. Wilson has agreed to defend the twins, and he focuses his efforts on matching the bloody fingerprint on the knife with one of the many prints he has collected from the town's women and girls. Wilson is convinced that the mysterious girl he had seen in Tom's room is behind the crime. Further, he refuses to suspect that Tom might be guilty, because: 1.) It was not in Tom to kill someone; 2.) even if it was, he would not turn his aggression to his uncle and benefactor; and 3.) it would be against Tom's own self-interest, because as long as the Judge was alive, Tom could rely on him for support and could try to win back the Judge's favor. If the Judge was dead, there was no way for Tom to get back into the torn up will. Though the will had in fact been revived, as far as Wilson knew, there was no way Tom could know that. Finally, Tom was in St. Louis when the murder occurred, and only learned about it when he read the newspaper.
The weeks drag by and the date for the twins' trial finally arrives. The State, with Pembroke Howard prosecuting, has a strong circumstantial case against Luigi and Angelo, and the convictions seem all but certain. When it comes time for Wilson to present the defense's case, he announces that he has only three witnesses - the Misses Clarkson - who will testify that they saw a veiled woman escaping the Judge's house through the back gate, a few minutes after Judge Driscoll's cries for help were heard. He states that this testimony, combined with other circumstantial evidence he will offer, will show that there is another person involved in this crime, who has not yet been found.
Despite feeling secure that his deeds cannot be detected, the early part of the trial has made Tom feel somewhat uneasy. However, once he sees just how weak Wilson's case is, all feelings of concern melt away. In fact, Tom feels so confident that he has escaped all liability that he decides to stop by Pudd'nhead Wilson's house that night to tease him about it. He finds Wilson in his house, still pouring over his glass slides, trying to match the knife prints to the fingerprints of some town woman. Tom notices a slide with Roxy's name marked on it, and picks it up to take a better look. In doing so, he inadvertently leaves his own fingerprint on the glass. When Wilson sees the print, he immediately recognizes it as the print from the knife. He chastises himself for not considering that the mysterious girl might be a man disguised in girls' clothing. After Tom leaves, Wilson takes out and examines the various fingerprints he collected from Tom over the years. This leaves him perplexed, as Tom's baby prints do not match his later records. Wilson goes to bed still confused, but when he awakes from a dream, the puzzle is solved.
Wilson spends the next morning enlarging his fingerprint records for use in court. At trial, he electrifies the town with his shocking revelations. He begins by accepting and even endorsing the State's proposition that the bloody fingerprints on the knife's handle belong to the culprit. The courtroom audience is stunned to hear Wilson make such a concession. Pudd'nhead then proceeds to explain the science of fingerprinting to the court. He states that fingerprints provide a personal "autograph" that can consistently identify a person throughout his or her lifetime. To demonstrate the accuracy of this science, Wilson has a number of town citizens - including Angelo and Luigi - press their fingers to the window while he is not looking. He then correctly matches the prints to their respective owners. To assure the audience that this was not merely luck, Wilson performs the feat a second time. With the courtroom now hanging intently on his every word, Wilson uses his enlarged prints to prove that the bloody fingerprints on the murder weapon do not even resemble those of the twins. This earns Wilson a thunderous crash of applause. Finally, Wilson makes two revelations that shock the townspeople and bring about Tom's demise: he man the town has come to know as Tom Driscoll is in fact the slave Valet de Chambre, switched in infancy by his mother, and this false heir is guilty of murdering Judge Driscoll. Roxy throws herself to her knees and begs for mercy, while the usurper is taken in custody.
Tom's true treachery and selfishness are revealed in these closing chapters. As we saw in the preceding chapters, Tom was willing to sell his own mother down the river to raise enough money to pay off his debts. Now, he is willing to steal from, and even kill, his benefactor, who took Tom in when his father died, treated Tom like his own son, and provided for Tom's every need. Moreover, after killing Judge Driscoll, Tom shows no remorse for what he has done, and is content allowing two innocent men go away for his crime.
We also see here that the code of honor, which the citizens of Dawson's Landing consider so critically important, is somewhat arbitrary. The duel between Judge Driscoll and Luigi - in which either man easily could have been killed by the other - was not only tolerated, it was celebrated. The combatants emerged from the duel heroes. Similarly, it was widely recognized and accepted after Judge Driscoll's speech lambasting the twins that Luigi was looking to challenge the Judge to another duel, and that upon sight, either man might kill the other. Yet, despite the general acceptance of killing in the name of honor, Luigi and Angelo are charged with murder when the town suspects that they are guilty of Judge Driscoll's death.
When the twins first arrived in Dawson's Landing, they were treated to a royal welcome and were the talk of the town. Their fame and reputation in the town were only magnified by Luigi's duel with Judge Driscoll. Yet, the town is quick to turn on the two Italians. Their standing in the community is first tarnished when Tom plants the seeds of doubt as to whether Luigi and Angelo ever possessed the Indian knife. By spreading the rumor that the Capello's offered reward is a hoax, Tom is able to paint the twins as dishonest charlatans. This attack on their reputation is compounded by Judge Driscoll's campaign to slander their good names and ensure their failure in the aldermanic election. When the two are suspected of murdering Dawson's Landing's leading citizen, the town's rejection of them is complete. Indeed, Aunt Patsy is the only person who comes to visit the twins while they are in jail awaiting trial. This fall from glory can be contrasted with Pudd'nhead Wilson's reputation. Upon first arriving, Wilson is ridiculed as a fool and isolated as an outsider. However, by the novel's end, he is a well-respected member of the community and is easily elected mayor.
Ultimately it is Tom's own arrogance that brings about his downfall. When he sees the weakness of Wilson's case, he cannot help but taunt Pudd'nhead. Rather than just being grateful for his good fortune and letting Luigi and Angelo take the fall for his crime, Tom goes to Wilson's house to rub it in his face. In doing so, Tom inadvertently provides Wilson with the one clue he needs to solve the mystery - his fingerprint. As Tom has previously foreshadowed, his hand proved to be his deadliest enemy.
In the end, when Wilson reveals Tom's true identity and names him as the murderer, Roxy appears to abandon her son. Falling to her knees, she begs for mercy for herself, but makes no plea for her child. Perhaps in betraying and selling her down the river, Tom has used up all of her maternal goodwill.