Mrs. Cheveley and Lord Goring confront each other, the former having come to trade Sir Robert's letter for Goring's hand in marriage. Their conversation reveals that during their courtship many years ago, Mrs. Cheveley seduced Goring to swindle him for a settlement. Now, however, Cheveley declares that Goring is the only man she has ever cared for. In any case, Goring refuses harshly.
Cheveley reasserts her intention to ruin Sir Robert, justifying her scheme as a mercenary transaction; Goring accuses her of defiling a sacred love, an act for which there can be no forgiveness. Coyly defending herself, Mrs. Cheveley protests that she only came to expose Sir Robert while visiting the Chilterns to retrieve her brooch. Goring then casually retrieves the bracelet from his writing table and, informing her that it can only be worn as a bracelet, suddenly clasps it on her arm. Calmly he reveals that he gave the brooch/bracelet years ago to a cousin as a wedding present and now knows that Mrs. Cheveley was guilty of its theft. Claiming that he has heard her confession, he will deliver her to the police unless she gives him Sir Robert's letter. Desperately, Mrs. Cheveley tries to rip the handcuff from her arm; Goring taunts that she cannot without releasing its secret spring.
Fearing arrest, Mrs. Cheveley surrenders the letter. As Goring burns it, she catches sight of Lady Chiltern's letter on the writing table. Asking Goring for a glass of water, she snatches the letter and puts it in her pocket. She then announces to Lord Goring that Gertrude's confession of love to her paramour has come into her hands. As her final act of vengeance, she plans to send it— misconstrued as a love letter addressed to Goring—to Sir Robert immediately. Goring moves to wrest it from her, with force if necessary; Mrs. Cheveley rings the bell and triumphantly has Phipps show her out. The act ends with Goring alone, lighting a cigarette and biting his lip.
In some sense the play's climatic moment, the face-off between An Ideal Husband's most active characters—Mrs. Cheveley and Lord Goring—is particularly rich. We will consider it in three parts. Continuing with the theme of marriage, we will first examine Mrs. Cheveley's attitudes toward courtship and conjugal life. Second, we will take up Mrs. Cheveley's unmasking as a "monster" and Goring's relation to the feminine. Finally, we will briefly consider the various objects that change hands in the course of this showdown.
It is in this scene, with her proposal to Goring, that Mrs. Cheveley—to use Lord Goring's term—most explicitly defiles married life. Already we know that she has ruthlessly wreaked havoc in the Chilterns' household; we now learn that she swindled Goring with a false courtship in their youth. Thus Mrs. Cheveley offers a condensed version of her philosophy of marriage in a clever epigram: "Romance should never begin with sentiment. It should begin with science and end with a settlement." Ever the opportunist, Cheveley would thus substitute the "settlement"—that is, financial gain—for romance's "sentiment." Accordingly, she makes her proposal to Goring a vulgar transaction, offering to trade Sir Robert's letter for his hand. At the same time, whether Cheveley truly still loves Goring is unclear: her uncharacteristic pauses after Goring's insults remain ambiguous.
Notably, Act III avenges these crimes against marriage through the diamond brooch. Revealed as a wedding gift Mrs. Cheveley stole in her youth, the brooch returns as evidence of a past crime, entrapping a woman who would manipulate another's past wrongs to her own advantage and ruin his conjugal bliss. The poetic justice in her arrest is clear.