Petulant and Witwoud finally show up with no clue about what’s going on, as usual. Mirabell reminds them that they once served as his witnesses to a certain legal document. Then, he calls them forward to examine the document they signed. The two agree that the document contains their signatures but they do not remember what they signed. Mirabell now begins to unveil his trump card, but before doing so, again reminds Wishfort of her promise.
Only fools like Petulant and Witwoud would serve as witnesses to a legal agreement without first reading what they are signing. Mirabell knew that they wouldn’t read the document, which is why he chose them as witnesses in the first place. In this way, he could keep the means of thwarting Fainall secret.
Mirabell addresses Fainall and informs him that before his wife married him, she signed away her fortune to Mirabell to prevent him from trying to wheedle it out of her. Mirabell continues to explain that he warned Arabella Languish (Mrs. Fainall) of Fainall’s bad temper and reputation. However, she was fond of Fainall and gave him the benefit of the doubt. But she did accept Mirabell’s advice and took precautions against her future husband, the parameters of which are contained in the binding legal document Mirabell holds.
Though Mirabell didn’t want to commit to Mrs. Fainall and marry her, he did feel a sense of obligation toward her as a good friend. All his behavior toward her once the affair is over is guided by this sense of duty, rather than love. Thus, his decision to take on the role of her secret legal adviser seems noble rather than cruel for remaining such an integral part of her life after the affair.
At first Fainall thinks that Mirabell is bluffing. But then he begins to read the document, and realizes that he has been outwitted and his case against Wishfort and her daughter is no longer valid. Mirabell continues that Arabella’s precautions are “the way of the world” with the “widows of the world.”
Mirabell’s outlook on the world turns out to be superior to Fainall’s because Mirabell understands that planning for the worst while giving people the option to act well is a better approach than simply thinking the worst about other people and then pre-emptively acting terribly toward them.
Enraged, Fainall charges at Mrs. Fainall and screams that he will get revenge. However, Wilfull steps in between them and blocks Fainall. Fainall shouts that Mirabell hasn’t heard the last of this. Arabella addresses Marwood and tells her that she looks so upset that she better vent her anger. Marwood, humiliated and defeated, swears that she’ll spend the rest of her life trying to exact revenge. Fainall and Marwood depart.
Wilfull’s manly bravado finally becomes useful when Fainall tries to attack his wife and Wilfull defends her. The once foolish Wilfull is maturing and his progress makes the decline of Marwood and Fainall seem even more pronounced. Meanwhile, Fainall and Marwood are now lost to a desire for revenge—they are doomed to loveless and trustless lives.