The scene, as before, is Lady Wishfort's house. Lady Wishfort has discovered Mirabell's plot. Foible tries unsuccessfully to make excuses for herself.
Fainall now makes his demands. As Millamant's fortune of 6,000 pounds was presumably forfeit when she refused to marry a suitor selected for her by Lady Wishfort, he wants the money as his price for not blackening his wife's reputation. He also wants the remainder of Mrs. Fainall's fortune turned over to his sole control. And he insists on Lady Wishfort's not marrying again so that he be sole heir. These terms are very harsh, and Lady Wishfort might not be prepared to go along with them except that Mrs. Marwood, standing by, goads her on by harping on the public disgrace of her daughter, Mrs. Fainall.
When the two maids now reveal that Fainall, in his turn, has been unfaithful to his wife, he refuses to be deterred; he is willing to be the subject of scandal himself, but he will still make public his wife's shame. When Millamant states that she is prepared to marry Sir Wilfull, thus meeting the wishes of her aunt and saving her 6,000 pounds, Fainall suspects a trick, but he can still demand control of the balance of his wife's estate, and now also the control of Lady Wishfort's. At this point, Mirabell presents the evidence which will protect Mrs. Fainall. At the time of her marriage, they had judged Fainall's character correctly, and Mrs. Fainall secretly signed over her fortune to Mirabell's control. There is, therefore, no money which Fainall can successfully obtain.
In great anger, Fainall and Mrs. Marwood leave the stage, vowing dire vengeance. Lady Wishfort, having discovered that Fainall was a villain and that Mrs. Marwood, her friend, was not a true friend, is now prepared to forgive Mirabell; Millamant can now marry him with her aunt's consent. It is on this happy but somewhat indeterminate note that the plays ends.
The fifth act is muddled; there is far too much plotting and action. Fainall comes in with his demands. Mirabell and Sir Wilfull Witwoud enter to frustrate part of them. Foible and Mincing disclose the information that Fainall and Mrs. Marwood have also been guilty of adultery. For the first time, we hear of the deed Mrs. Fainall signed. And, finally, Lady Wishfort forgives everyone.
If one looks at it structurally, it is possible to see that Mirabell's original scheme is here balanced by the counterplot of Fainall and Mrs. Marwood. They, in turn, are foiled by Foible and Mincing on the one hand, and by Millamant's presumed willingness to marry Sir Wilfull on the other. But these developments are then countered by Fainall's insistence on the balance of Mrs. Fainall's money. And this move is conclusively countered by Mirabell's producing the deed signed before Mrs. Fainall's marriage, presumably in anticipation of, and protection against, just such a situation.
Lady Wishfort, in this act, becomes almost a sympathetic character. Her faults and her vanities are many, but here we see her trying to protect her daughter, finding that the people whom she trusted have proven completely treacherous. Caught on the one hand by the desire to save those whom she loves, and trapped by the treachery of those she trusted, she is an odd figure in a very unusual situation in Restoration drama.
The ending of the play is not entirely satisfactory. For one thing, one is finally left with the question, "What of Mrs. Fainall?" She will retain her money, but her lover is lost to her, and it is not entirely clear that Fainall and Mrs. Marwood will not find some rather unpleasant revenge.