Moll is growing into a very beautiful young woman, and she becomes vain of her appearance. The two sons of her adopted family begin to take notice of Moll (who at this time is known as "Mrs. Betty"). The eldest son is of a worldly and dissolute character. He flatters and flirts with Moll and eventually seduces her--which, as Moll confesses, was actually not all that difficult a task. They become regular lovers, and he gives her quite a bit of money in exchange for her sexual favors. She believes, however, that he means to marry her, and so she is bewildered when the younger brother, Robert (also called "Robin"), makes her a marriage proposal as well. Robert, captivated by Moll's beauty, wants to wed her immediately and without regard for the certain disapproval of his family and friends. Because he makes no secret of his desires, his mother and sisters start to treat Moll gruffly and even begin to talk of turning her out of the house.
Moll consults with the elder brother about how to handle the situation. Much to her surprise, her current lover encourages her to accept Robert's offer. He obviously sees this marriage as an easy way of extricating himself from a potentially embarrassing liaison. Moll, however, is aghast at this suggestion; she feels herself bonded to the elder brother indissolubly, and she admonishes him "to remember the long Discourse you have had with me, and the many Hours pains you have taken to perswade me to believe myself an honest Woman, that I was your Wife intentionally, though not in the eye of the World, and that it was as effectual a Marriage that had pass'd between us as if we had been publickly Wedded by the Parson of the Parish." She realizes that if she marries the younger brother, she will have been nothing but a prostitute to the elder: "If I have been perswaded to believe that I am really, and in the Essence of the Thing your Wife, shall I now give the Lye to all those Arguments, and call myself your Whore, or Mistress, which is the same thing?"
The shock of this whole series of developments throws Moll into a fever, from which she takes five weeks to recover. The family's concern over their younger son's attachment to Moll becomes increasingly obvious during this period, and they interrogate her repeatedly about his advances and her own intentions. She first claims that Robert is not serious, and then declares that she would never marry him against the family's wishes. Robert presses his family for their consent, believing that then Moll will marry him. His older brother aids him in this campaign, urging both Moll and his mother to agree to the marriage. He tries to work on Moll without having to violate his promises explicitly, but finally he makes her understand that he will have nothing more to do with her, whether she marries Robert or not. She begins to see the true contours of the situation, and when the mother eventually consents, she agrees to marry Robert. The older brother arranges things so that Robert is in too much of "a Fuddle" on his wedding night to know that his bride is not a virgin. Moll has no love for Robert and continues to cherish a flame for her first lover. Her husband dies after five years, and their two children are sent to live with Robert's parents.
The situation in which Moll eventually finds herself--in love with one brother but compelled to marry the other--is the stuff of tragedy. Defoe gives the plot a fairly comic treatment, however, utilizing the episode mainly to demonstrate Moll's early naiveté and to show her perseverance and her quickness to learn from her experiences. Moll singles out the growth of her youthful vanity as marking a turning point in her life. Up to this point, Moll has had nothing to reproach herself with except a childish ignorance. "Thus far I have had a smooth Story to tell of myself, and in all this Part of my Life, I not only had the Reputation of living in a very good Family,...but I had the Character too of a very sober, modest, and virtuous young Woman, and such I had always been; neither had I yet any occasion to think of any thing else, or to know what a Temptation to Wickedness meant." Yet the narrator backs off of the sermon on the evils of vanity, or at least she recasts those evils in material, not spiritual terms. The lesson she draws is one of expediency rather than of piety. When she warns her younger readers "to Guard themselves against the Mischiefs which attend an early Knowledge of their own Beauty," the mischief to which she refers is not immoral sexual behavior but rather the credulousness that will allow a woman to be the dupe of a more sophisticated man. She admonishes herself for her lack of attention to practical matters--not for the fact that she yielded to temptation, but for the fact that she failed to secure her own interests as she might have.
The scene of Moll's seduction is one of the book's raciest episodes. As the heroine becomes more sexually experienced, the narrator ceases to present the sexual facts of her story with the same romance and titillation. Desire and emotion are in fact conspicuously minimized in this novel, which distills human existence to its economic and materialistic bottom line. The emotional responses of the character Moll contrast markedly here with the wizened perspective of the septuagenarian who narrates the story. As Moll grows into her adult self, this divided perspective closes somewhat: she matures into a pattern in which her first reactions to events, which may be emotional or impetuous initially, quickly resolve into stoic and pragmatic courses of action.
Yet the gap between the narrator and the protagonist remains important throughout, serving to reinforce the conditional morality that the book so often propounds. Life decisions in Defoe's novel cannot be divorced from the circumstances under which they are made. The narrator's most frequent strategy in commenting on her own life is to imagine herself into her former situation, rather than to impose the wisdom of her years on her earlier experience. Moll's ability to perform this imaginative displacement is part of what enables her to tell her story with such tenderness of sympathy and understanding. The narrator is never coy with her reader, which is part of her appeal. She presents her own responses and motivations frankly and unabashedly, as when she confesses that she was too pleased with her first lover's attentions to resist him. The fact that we get no real external perspective on Moll's life, however, limits the capacity of the novel to pronounce any stern judgment or to come to an objective moral resolution, and many readers find it difficult to discern even the author's own real opinion of Moll's character.