The setting is once again confined to Lady Wishfort's house.
Lady Wishfort has discovered Mirabell's plot. She denounces Foible for being Mirabell's accessory and turns her out, charging her with ingratitude and reminding her that she saved her from the streets. Foible pleads for mercy. She claims that Mirabell seduced her. After all, she says, she was not the first one to succumb to his lies. He had assured her that Lady Wishfort would come to no harm. Foible protests that Waitwell could not have consummated his marriage with Lady Wishfort, since it would have been illegal: he was already married to her (Foible). This explanation serves only to aggravate Lady Wishfort further, and she leaves, threatening to have Foible arrested.
Mrs. Fainall enters, and Foible confides her pitiable condition to her. She laments that Lady Wishfort has gone to fetch a constable to arrest her and that Waitwell is already in prison.
Mrs. Fainall consoles Foible by telling her that Mirabell has already gone to free Waitwell on bail. She tells Foible that this is all the fault of Mrs. Marwood and Fainall. Foible confirms this and tells her that Fainall had Waitwell arrested when he pretended to go out for the papers, and Mrs. Marwood revealed Mirabell's plot to Lady Wishfort. Mrs. Fainall is relieved to learn that her mother did not read that part of the letter which stated her involvement in Mirabell's plot. But Mrs. Marwood has revealed Mrs. Fainall's affair with Mirabell, and now the Fainalls' marriage is at an end. However, Foible reveals that she has some confidential information which may be used against Mrs. Marwood.
At this point Mincing enters with the news that Mirabell has freed Waitwell from custody and that Mirabell wants to speak to Foible. She also says that Mirabell has instructed Foible to hide until Lady Wishfort's anger subsides. Mincing further reveals that Fainall has put his plan into action and has threatened to divorce Mrs. Fainall if she does not hand over her entire fortune immediately. Mincing finally says that Millamant has resolved to marry Sir Wilfull Witwoud in order not to lose her fortune of six thousand pounds. Mrs. Fainall asks Mincing to swear to the truth of her statement when she is called for. Mincing agrees and leaves, along with Foible.
Lady Wishfort, entering with Mrs. Marwood, expresses her gratitude for her friend's loyalty. First, she alerted Lady Wishfort about Mirabell's false vows of love. She also revealed Waitwell's posturing as Sir Rowland. And finally, she interceded with Fainall to save the family reputation, which her daughter's (Mrs. Fainall's) misdeeds had damaged. Lady Wishfort reproaches her daughter for her transgressions, because of which she will now have to pawn her plates and jewels. Mrs. Fainall protests that she has been falsely accused and implies that she knows about Mrs. Marwood's relationship with Fainall. She challenges Mrs. Marwood to prove all the allegations made against her. She then accuses Mrs. Marwood of being a false friend to Lady Wishfort and leaves, claiming that she can prove her innocence.
After her daughter leaves, Lady Wishfort considers letting her daughter prove her innocence. She claims that her daughter has had an impeccable and virtuous upbringing. Lady Wishfort reminisces that she had created aversion to men in her daughter early in childhood. She had never been permitted to play with boys and even all her dolls were of the feminine gender. She had never looked a man in the face, apart from her own father and the chaplain. She cannot believe that her daughter would indulge in an extramarital affair. She says that Fainall should prove that he has been cuckolded. But Mrs. Marwood reminds her that it would tarnish the reputation of the family if the matter were to be heard in a public court of law. It would cause a scandal and become a joke in the town.