Roxy finds Tom in such a miserable state that her heart is touched and her maternal instincts rise. She tells her son that she loves him, and Tom winces in response. Even though he has black blood in him, this "was far from reconciling him to that despised race." Roxy then comes up with yet another plan to save her son from ruin. She says she is worth six hundred dollars, and offers to let him sell her back into slavery. She says, "Ain't you my chile? En does you know anything dat a mother won't do for her chile?"
The plan, as Roxy originally intends it, is for Tom to find a farmer up country (where conditions for slaves are better) and to sell Roxy there, using the proceeds to pay off his outstanding gambling debts. Then, after a year has passed, Tom is to buy her back into freedom. However, Tom betrays his own mother and sells her to a cotton planter in Arkansas - or in other words, "down the river," where slave conditions are especially harsh. He justifies his deception by convincing himself that Roxy won't even know where she is and that by the time she figures it out, she'll have become accustomed to her surroundings. Moreover, she will have the knowledge that the slavery is merely temporary and that she will get her freedom back in one short year. Tom's action is the basest form of treachery, for "by voluntarily going into slavery - slavery of any kind, mild or severe, or of any duration, brief or long - was making a sacrifice for him compared with which death would have been a poor and commonplace one."
Roxy is taken away from St. Louis aboard a boat. Having worked so many years as a chambermaid, Roxy is quite familiar with the operation of a steamboat. Consequently, she quickly deciphers that the ship is traveling downstream with the current. She laments, "I's sole down de river!"
The election season gets underway, and the Italian twins throw all their efforts into their campaign for the alderman board. However, their campaign suffers for a number of reasons. First, Luigi and Angelo were too popular, and thus experienced some natural backlash. Secondly, speculation was spreading that the reward for the lost knife was just a hoax. The twins are relying on this election to restore their good names, and thus work even harder to succeed. However, Judge Driscoll and his nephew work just as hard, if not harder, to undermine the twins' efforts. The final straw comes when the Judge delivers a speech to the town. He indicates his belief that the reward for the stolen knife is merely "humbug and buncombe, and that its owner would know where to find it whenever he should have occasion to assassinate somebody." This speech causes a tremendous commotion in the crowd and throughout the town. Pudd'nhead Wilson is elected mayor, while Luigi and Angelo are soundly defeated. The friendless twins withdraw from public life to suffer their humiliation in solitude. Judge Driscoll has been left "prostrated" by his efforts, but there are rumors that he'll be receiving a challenge from Luigi as soon as the Judge is well again.
After escaping from the Arkansas plantation, Roxy returns to St. Louis as a runaway slave. Disguised in men's clothing and with a blackened face, she confronts Tom. However, she is too heartbroken and worn out to storm around in anger. She tells her son of her ordeal: Though her master was a decent man, his wife (a Yank) was jealous of Roxy's beauty. As a result, the wife made sure that the overseer was particularly harsh on Roxy, who often suffered lashes because she could not keep up with the work of the other slaves.
One day, Roxy witnessed the overseer thrashing a young slave child who had stolen some food. She took the overseer's stick away from him and struck him with it. Roxy then escaped on a horse and rode down to the river, where she planned to drown herself, rather than risk being caught and put back to work. As luck should have it, Roxy recognized a passing steamboat as a vessel on which she used to work as a chambermaid. She climbed aboard the ship and her friends provided her with passage to St. Louis. When she arrived, she recognized her master, who was passing out and posting runaway slave notices in town.
Roxy now interrogates her son to determine whether the Arkansas farmer has been to see him. The truth is that the farmer has in fact paid Tom a visit, and told Tom that he suspected that there was something dubious about the sale of Roxy. Tom had been confident, however, that that his mother's maternal instinct would prevent her from ever coming to St. Louis, as she would surely be aware of how much trouble this would get him into. Now that she has disappointed him with her arrival, Tom feels that he has no choice but to turn her over to the farmer, or else he will be forced to repay him (a proposition he cannot afford). Roxy is too clever for her son and figures he must have promised the master that he would assist in capturing her. Roxy then orders Tom to go to his uncle, confess what he has done, and get the money to purchase her freedom. She threatens that if he fails to do so, she will go to the Judge herself. Tom gives in and agrees to do it, though he privately decides that rather than confessing to his uncle, he will merely steal from him.
In Chapter 16, Roxy offers to make the ultimate sacrifice for her son and demonstrates just how deeply she loves him. Herein lies the novel's true tragedy. Not only does Tom not appreciate or reciprocate his mother's affection, he betrays her at precisely the moment she is giving up everything for him.
When Roxy sees the miserable state Tom is in, her maternal instinct takes over. All of her disappointment for his cowardice and failures and all of her resentment for the years of cruel treatment are washed away. All that remains is a mother's love. Yet, this love is repulsive to Tom, who still views blacks as a despised race. We see here that "nurture" remains triumphant over "nature." Learning of his black blood and slave origins is not enough to truly transform Tom's world view. In his mind, he is still a wealthy slaveholder and blacks (including his mother) are mere property.
The true demonstration of motherly love comes when Roxy offers to forfeit her freedom and be sold back into slavery to pay off her son's debts. As Twain writes, to return to slavery - even temporarily - is a greater sacrifice even than giving up one's life. But at the same time that Roxy demonstrates her love and courage, Tom shows his selfishness and true cowardice. Resorting to the law, rather than a duel, to settle a grievance is only cowardly according to the arcane rules of Judge Driscoll's honor code. However, it would likely be universally agreed that a person who rather than facing up to his failures and paying off his own debts sells his own mother into slavery, and in doing so betrays her by sending her to the one place she dreads most (down the river), is truly a coward.
Tom continues to show this cowardice, even after Roxy escapes from the plantation. After learning of the plantation's harsh conditions and the cruel treatment his mother suffered at the hands of the overseer and the master's wife, Tom's concern is not for Roxy's well being. Instead, he is concerned for his own hide, as he fears that the master will learn that the sale was a sham, leading to even deeper trouble for Tom. As such, Tom plans to return his mother into this terrible captivity. Moreover, we see that he takes Roxy's love for him for granted. When he first learns that Roxy has escaped, he tells himself that his mother loves him too much to return to St. Louis and risk getting him into deeper waters. It never occurs to Tom that after betraying Roxy and selling her down the river, her immediate concern may no longer be his welfare.