The narrative shifts back in time, to just after Roxana's marriage, and returns to the subject of Roxana providing for her children. One day, while Amy was visiting the son, his two sisters end up being there as well (the other daughter has also been found and provided for, although details are not given). Amy confirms that she is their benefactress, but that she is not their mother. Susan (the daughter who previously worked as a maid in Roxana's household) is very upset, and insists that Amy must be her mother. She also hints that she can protect any secrets that need to be kept. Amy is very worried, and hurries back to Roxana. They decide to try and find out what Susan knows, and also to try and keep her calm. Amy returns to see Susan, insists that she is not her mother, and tries to get her to calm down. Susan is very emotional and agitated, and reveals that she has pieced together a significant amount of her mother's history. She knows that her mother was in a relationship with the Landlord/Jeweller, and went away to France with him; then, having known that Amy came back from France with Lady Roxana and that Amy acted as her benefactress, that Amy must be her mother. Amy continues to deny this, but then Susan latches on to the idea that in that case, the Lady Roxana must be her mother. She also hints that even though no one knows the whereabouts of Lady Roxana, she would be able to find her.
Amy is furious and terrified by all of this, and tells Roxana that maybe they should murder Susan. Roxana is horrified by this suggestion, but she is also very agitated by the situation. She is close to going to Holland and enjoying life with her new husband, but everything will be ruined if she is revealed to have been the famous courtesan. Amy threatens to cut off Susan's money if she insists on pursuing her theory, but Susan ignores her, and insists that she is going to find Roxana; she also reveals that she knows Roxana has just gotten married. Amy becomes even more worried, and tells Roxana that they really should kill Susan, but Roxana refuses. Susan also persists, even after her sister, aunt, and uncle all tell her to stop making trouble, lest they lose their financial patronage. By this point, Roxana is anxious to get to Holland as quickly and discreetly as possible. She arranges for she and the Merchant to sail on a private ship, with no other passengers. The captain of the ship invites them to have dinner on the ship with him and his wife before they set sail.
When Roxana, the Merchant, and the Quaker arrive at the dinner, Susan is there. After she began attending school, she became a very close friend of a genteel young woman, who then married a ship captain. Therefore, Susan is attending the dinner along with her friend and her friend's husband. Roxana is terrified of being found out, but it seems that Susan doesn't recognize her. She is worried, however, because Susan comments on her looking familiar, and now needs to get out of the voyage, because it will be too dangerous to be aboard an extended voyage with Susan. After the dinner, Roxana has the Quaker visit with the Captain's wife, and explain that Roxana is pregnant and ill, and may have to delay the voyage. However, the Quaker reports to Roxana that Susan (who was present during the meeting) kept asking questions about her. A few days later, Susan and the Captain's wife also unexpectedly drop by to visit Roxana at the Quaker's house; they barely have time to hide Amy. During the visit, Susan keeps commenting on how much Roxana looks like a lady she knows (ie the Lady Roxana), and also notices that Roxana's casual nightwear reminds her of the Turkish costume she once saw the Lady Roxana wearing.
Susan begins telling stories about her time working in the household of the Lady Roxana; the Quaker is interested, because she has heard about the famous courtesan (although of course she has no idea that this is the same person who has been living with her for years). When Susan tells the story about seeing the Lady Roxana in her Turkish dress, the Quaker mentions that Roxana has a similar outfit; fortunately, Roxana claims that her outfit is packed up, so she cannot show it off, and Susan and the Captain's wife finally leave. Afterwards, the Quaker notices that Susan seemed oddly fixated on something, and can't help noticing that a lot of the things Susan said about Lady Roxana were similar to what Roxana has shared about her past before she came to live with the Quaker. Roxana tells her husband that she is ill and possibly pregnant, and he readily agrees to postpone the trip to Holland. He breaks the news to the Captain, but comes home with some gossip that the Captain overheard, implying that Susan knows that Roxana is her mother. By this point, Amy and Roxana are both terrified of being found out.
Roxana persuades her husband to get out of London with her; she wants to make sure he doesn't spend any more time with the Captain, and hear anything else. She also brings Amy with her because she is afraid that Amy might kill Susan if she is left in London; by now, Roxana wishes that Susan would die, but she cannot bear the idea of her being murdered. Roxana also leaves instructions with the Quaker in case Susan stops by; sure enough, Susan does show up at the house of the Quaker, asking for Roxana. The Quaker says that Roxana has left, and she doesn't know her whereabouts. Susan explains to the Quaker all about why she is convinced that Roxana is her mother and why she is trying to find her. Fortunately, the Quaker argues about why this isn't possible, but Susan won't listen to her. Susan also claims that she will never stop searching for her mother, and can think of several places where she might be. The Quaker loyally reports all of this to Roxana via Amy, while ensuring that the Merchant doesn't know anything about what is happening.
Amy begins trying to seek Susan out, and finally runs in to her just as she is about to board a boat to cross the Thames. Amy misleads Susan by implying that she is now living in Greenwich, and the two of them sail together. Amy tries to rebuke Susan but Susan by now reveals that she knows her mother's name, and the name of her husband, and it is just a question of finding out where they are. The two of them carry on arguing for hours in a park in Greenwich, with Susan admitting that she is suspicious that Amy will hurt her. Amy says that she is no longer going to support Susan's brother or sister, and that Susan will have to live with the knowledge that it is her fault. When Roxana hears about this encounter, she moves again to an even more secluded location; the Quaker tells her that only a few days later, Susan made her way to the town where Roxana had been staying. She is also so angry about Amy's insistence on murdering Susan that she sends Amy away and says that she wants nothing more to do with her.
Fairly soon afterwards, Roxana begins to miss Amy. She decides to hasten the departure for Holland, telling her husband that she is not pregnant after all. Tensions are still running high, with Susan stalking the Quaker's door, and even following the Quaker when she goes to visit Roxana. One day, Roxana receives a letter from the Quaker informing her that Amy has visited her. During the visit, Amy explained that she took it upon herself to make sure that Susan wouldn't bother them anymore; the Quaker also notes that she hasn't seen Susan for a few weeks. The Quaker naively assumes that Susan must have given up and left London, but Roxana is convinced that this means Amy has killed Susan. She tries to track down Susan, and the last she hears is that Susan was staying with family, went out in a coach with a woman matching Amy's description, and hasn't been seen since. Roxana is overwhelmed with guilt and horror.
Roxana prepares to go to Holland, and without Amy, she entrusts the Quaker to help her, although she never fully reveals all of her secrets. Roxana has the Quaker make arrangements for the ongoing financial support of her other daughter, acting now in Amy's stead under the guise that Amy has gone back to the Indies. Roxana even gets the chance to see her daughter a few times when she comes to the Quaker's house. She also tells the Quaker that if Amy turns up, she would be willing to have her back if Amy can promise that she did not hurt Susan. However, Amy ends up back with her in Holland without providing this assurance. Roxana's life in Holland begins happily, as she has previously described, but she hints that she was eventually punished for his misdeeds, and ended up losing everything.
Roxana's decision to look for her children, and then provide for them financially is a caring, but also risky choice. Susan in particular poses a threat because unlike the others, she might be able to connect Roxana to the famous courtesan whom she once worked for. What will become particularly threatening about the situation is that Susan is not unlike Roxana herself: intelligent, observant, stubborn, and determined to the point of obsession. It is hinted at one point that Susan is Roxana's namesake, which heightens the idea of a similarity, and furthers the motif of doubling in the novel. For much of the novel, it has been Amy who functioned as Roxana's dark double, but the introduction of Susan puts all three women in to a complicated triangle of jockeying for power and control. Part of why Amy might be so willing to want to see Susan vanish, even if it means killing her, is that Susan threatens to disrupt the bond between Amy and Roxana, and also the control Amy can exert over her mistress. Amy has almost total control over Roxana's psyche because she is the only one who truly knows Roxana, but if Susan becomes part of Roxana's life, that bond might be superseded. Roxana feels a strong tie to Susan, referring to her as flesh and blood, and Susan could be the one person she feels more kindship with than Amy. To add another layer, although the plot point is never mentioned again in the novel, Amy has previously given birth to a daughter (fathered by the Landlord), who would not be so much younger than Susan. The return of Susan is a form of the return of repressed crimes and broken maternal bonds for women.
In a novel where characters often seem quite limited in their emotional range, Susan distinguishes herself with her strong emotional appeals. She is bitterly hurt by her suspicion that her mother is alive, knows her, and refuses to claim her. Unlike Roxana, Susan also doesn't seem to be appeased by money and stability. Her guardians and her sister and brother all seem willing to avoid asking uncomfortable questions so long as they continue to reap the financial rewards of their mysterious benefactor, but Susan doesn't care about losing all of the money if it means that she can find her mother. Susan is just as stubbornly indifferent to money as her mother is obsessed with it, which is interesting given the reversals of their experiences. Roxana grew up with money, lost it all, and then becomes obsessed with gaining it back, and then some, whereas Susan grows up in poverty, becomes abruptly wealthy, and then continues to be indifferent to the money. While Susan seems to start by wanting to simply be known and claimed by her mother, her obsession seems to grow darker. She may even want to pursue Roxana in order to destroy her, although Roxana and Amy seem to project their own emotions onto Susan and assume that she is out to get them. Because they are driven by their guilt, they assume that she is coming to punish them.
Susan is entirely successful at driving a rift between Amy and Roxana, and in fact achieves the closest they ever come to parting ways. Throughout the novel, Amy has been presented as somewhat amoral, and willing to do whatever is necessary to survive and protect herself. Still, both Roxana and the reader are shocked when Amy openly proposes killing Susan. Roxana desperately wants Susan to leave her alone, but she cannot imagine harming her own child. Everything Roxana has done has been in hope of protecting her children and possibly even establishing a relationship with them, so it feels like a complete betrayal to destroy one of them. Given the intense connection that Roxana feels to the child she has birthed and nursed, killing her daughter would be like killing her own self. Throughout the novel, Roxana has repeatedly reneged on her moral principles and done things she never thought she would do, but murder is a bridge she will not cross. For the first time, Amy cannot talk her mistress into taking her side, and Roxana is eventually so disgusted that she sends Amy away. However, a keen reader will notice that, given that Roxana is afraid of Amy murdering Susan, it seems somewhat suspicious that she sends her away. She is actually giving Amy more freedom and more opportunity to hurt Susan, which, chillingly, Amy ends up using.
Roxana's conviction that Amy has killed Susan reflects how tightly intertwined the two women are, and how Roxana has become steeped in cynicism. In contrast to the Quaker, who has an optimistic view of human nature, Roxana has now been corrupted to the point that she assumes the worst in people. Roxana also feels complicit in this horrible crime: even though she tried to forbid Amy from killing Susan, she knows deep down that she didn't do much to prevent it. Her own refusal to claim Susan, thereby risking her marriage and her fortune, also directly led to the crisis: she could have just told the truth, and that Amy would have had nothing to gain from the crime. This loss parallels Roxana's earlier loss of her children: she lost her daughter, found her, and then lost her again because she was not willing to risk everything that she had acquired. Roxana's narrative is ultimately one of moral ruin, and this explains why she recounts her life as a series of tragic missteps leading up to one great collapse.
For reasons that are not fully clear, Roxana does take Amy back, even though Amy won't even attempt to pretend that she didn't kill Susan. The two women only have each other to be truly honest with, and now that they know each other's darkest secret, they are bound more tightly than ever. Roxana's internal guilt would have been tormenting enough, but she hints at the end of the novel that she and Amy suffered some sort of terrible punishment for their crime. It might also be a mercy that Roxana's external reality ends up corresponding to her tormented soul, so she can at least end up knowing that she is paying the dues for her choices. Throughout the novel, Defoe grapples with the tension between a world dominated by Christian morality and the importance of making choices with integrity, and a dawning world order in which individuals profit by acting in their own self-interest. Roxana ends up choosing the wrong priorities in her life, and paying the price.