David and Mr. Peggotty continue to follow Martha into Westminster, where she approaches the riverside. The neighborhood is wet, impoverished, and littered with detritus from boats. To David, it looks as though Martha were "a part of the refuse [the Thames] had cast out, and left to corruption and decay." As Martha gazes out over the river, David approaches her in a state of "dread." She is talking to herself and looks so "wild," he is afraid she will jump into the water.
The place where Mr. Peggotty and David meet Martha is the first glimpse Dickens offers in David Copperfield of the abject poverty of parts of nineteenth-century London. The rot and pollution of the area, and Martha's association with this moral "corruption and decay," picks up on earlier imagery depicting sexual promiscuity as an infectious disease. In this case, however, the implication is that poverty taints whatever it touches, breeding prostitution and crime.
David calls out to Martha, who at first struggles to get away. When she sees Mr. Peggotty, however, she lets the men take her away from the water, although she continues to cry out "Oh, the river!" David tries to calm her down, but she says that she is drawn to the river, which is "like her" in the sense that it grows polluted and "defiled" on its way to the sea. Mr. Peggotty is horrified, and David—despite his own discomfort—tries to assure him that Martha doesn't mean what she says.
Martha's speech further develops the idea that her fallenness is a kind of pollution linked to social problems like poverty. She says, for instance, that she feels an affinity with the river that becomes "defiled and miserable" as it flows through London—possibly a reference to the role that urbanization was thought to play in encouraging prostitution.
Once Martha has stopped crying, David asks her if she knows who he and Mr. Peggotty are, and if she's willing to talk to them about little Em’ly. She says that she is, but wants to be sure that Mr. Peggotty doesn't consider her responsible for what happened. David assures her that he doesn't, and Martha says that she would have already killed herself if she had helped ruin someone who was so kind to her. In fact, she says, the worst thing about her own fall was that it cut her off from Emily's friendship, and when she learned Emily had run away, she would have done anything to undo it. Breaking down again, she begs Mr. Peggotty to kill her rather than to think the worst of her: she knows she is still far beneath Emily, but she nevertheless feels deep gratitude to her.
Martha's fear that Mr. Peggotty will hold her responsible for Emily's actions again implies that her mere presence could have somehow infected Emily. The fact that Martha is so horrified by this idea is a sign of the novel's relatively enlightened view on the plight of fallen women; contrary to popular wisdom, Martha's actions have not affected her basic sense of right and wrong. It's also noteworthy that Martha considers Emily to be her moral superior even now. The strong implication is that Martha has actually been supporting herself through prostitution, which would have been even more morally objectionable than simply having an affair out of wedlock.
Mr. Peggotty reassures Martha that he is in no position to judge her. He says he knows that she, like little Em'ly, was orphaned, and asks her to imagine Emily's feelings toward her adoptive father now—that is, that she both longs to see Mr. Peggotty again and is ashamed to go to him. Finally, he explains that he and David believe that little Em'ly will come to London, and asks Martha to help them find her when she does. Martha is at first surprised that Mr. Peggotty would trust her, but then solemnly pledges to do everything she can to find and help Emily, and asks "help, human and Divine, [to] renounce her evermore" if she abandons the task.
Like many other characters in the book, Martha is the product of a broken home. In fact, Mr. Peggotty implies that Martha was left entirely on her own after her parents' deaths, with no adoptive family to take care of her. This lack of guidance, Dickens suggests, likely played a role in Martha's later missteps. For that reason, the connection Mr. Peggotty establishes with Martha in this scene is especially significant; it is the first step toward drawing Martha into a surrogate family and therefore toward redemption.
David and Mr. Peggotty tell Martha everything they know of little Em'ly's whereabouts, and provide her with their addresses so that she can contact them. Martha refuses, however, to tell them where she lives, and also declines David's offer of money. Instead, she says that she will try to find work, because in accepting money, she feels she would be losing David and Mr. Peggotty's "trust." This, she says, is the only thing stopping her from killing herself. David begs her to put aside the idea of suicide, but all Martha can say is that she feels, for the first time, as though she has some kind of purpose in life—but she doesn't dare yet hope that she can be "saved." She then leaves, although David and Mr. Peggotty follow her until she reaches a more populous area.
Martha's relief in finally having a meaningful goal to pursue underscores the relationship between vocation and moral character in the novel: characters like Steerforth, who lack a clear sense of purpose and an ability to follow through on it are immature at best and immoral at worst. Part of what "saves" Martha is therefore having a form of work to pursue, but it's also important, by Martha's own admission, that this work be unpaid. Since Martha has likely traded sex for money in the past, either formally or (like Emily) as a kept mistress, payment itself has become suspect; the novel implies that she can only redeem herself by pursuing an entirely selfless goal.
David says goodbye to Mr. Peggotty and heads home. When he passes Miss Betsey's cottage, however, he notices that the door is open and goes to investigate, only to find a man standing in the garden eating and drinking. David realizes that it is the same man his aunt had stopped to talk to in the street years earlier. Eventually, Miss Betsey emerges from the cottage and tries to hand the man some money. She insists that it is all she can give him, but he refuses to leave without more. Upset, Miss Betsey asks why he treats her this way, and how she can "free [her]self" from him short of leaving him "to [his] deserts." The man asks why she doesn't abandon him, then, and Miss Betsey wonders aloud how hard-hearted he must be to ask her that.
Miss Betsey's relationship with her former husband reveals a more vulnerable (and therefore conventionally feminine) side to her. This vulnerability is evident not only in the fact that she clearly fears the man, but also in the fact that she still feels some level of affection for him. Although Miss Betsey recognizes that this tenderness is foolish and mostly the effect of nostalgia, she can't bring herself to cut her husband off entirely.
Miss Betsey and the man continue to argue, Miss Betsey insisting that she can't spare any more money and can't bear "seeing what [he has] become" any longer." She then begins crying, accusing him of treating her cruelly and turning her into a guarded and mistrustful woman. Taken aback by this, the man finally leaves, and David approaches his aunt, asking for permission to speak to the man. Miss Betsey, however, simply tells David to come in and give her a few moments to compose herself.
One reason why Miss Betsey was presumably so wary of David's infatuation with Dora is that she has experienced the effects of an ill-advised marriage firsthand. However, for Miss Betsey (and the novel's other female characters) the consequences of this kind of mistake are much more serious: since a middle-class woman's entire existence revolved around her husband and family, marrying the wrong person could ruin her entire life. Compared to Clara Copperfield, Miss Betsey is lucky in that she has managed to extricate herself from an abusive relationship, but the experience permanently altered her character.
David does as Miss Betsey asks, and she then tells him that the man is her husband, who is only "dead to [her]." She was deeply in love with him at one point, but he "repaid her by breaking her fortune, and nearly breaking her heart." This, she explains, is why she is so tough and unsentimental now. Nevertheless, she says, she tried to provide for him when the marriage ended, although his treatment of her meant that she could have won a "separation on easy terms." He squandered the money she gave him, however, and eventually married another woman. She does not want to see him arrested, however, so she continues to pay him money what she can. Once she has explained all this, Miss Betsey asks David not to speak any more about her "grumpy, frumpy story."
Divorce was difficult to obtain in nineteenth-century England, although it gradually became easier as the century went on. At the time Dickens wrote David Copperfield, however, a woman couldn't actually separate from her husband on grounds of abuse alone, so it's likely that Miss Betsey’s husband also had an affair. Regardless, his treatment of Miss Betsey reveals the vulnerable position of married women in the nineteenth century; he was able to squander her money, for instance, because a husband gained possession of all his wife's property on marrying her.