Before the action of the play begins, the following events are assumed to have taken place.
Mirabell, a young man-about-town, apparently not a man of great wealth, has had an affair with Mrs. Fainall, the widowed daughter of Lady Wishfort. To protect her from scandal in the event of pregnancy, he has helped engineer her marriage to Mr. Fainall, a man whom he feels to be of sufficiently good reputation to constitute a respectable match, but not a man of such virtue that tricking him would be unfair. Fainall, for his part, married the young widow because he coveted her fortune to support his amour with Mrs. Marwood. In time, the liaison between Mirabell and Mrs. Fainall ended (although this is not explicitly stated), and Mirabell found himself in love with Millamant, the niece and ward of Lady Wish-fort, and the cousin of his former mistress.
There are, however, financial complications. Half of Millamant's fortune was under her own control, but the other half, 6,000 pounds, was controlled by Lady Wishfort, to be turned over to Millamant if she married a suitor approved by her aunt. Unfortunately, Mirabell had earlier offended Lady Wishfort; she had misinterpreted his flattery as love.
Mirabell, therefore, has contrived an elaborate scheme. He has arranged for a pretended uncle (his valet, Waitwell) to woo and win Lady Wishfort. Then Mirabell intends to reveal the actual status of the successful wooer and obtain her consent to his marriage to Millamant by rescuing her from this misalliance. Waitwell was to marry Foible, Lady Wishfort's maid, before the masquerade so that he might not decide to hold Lady Wishfort to her contract; Mirabell is too much a man of his time to trust anyone in matters of money or love. Millamant is aware of the plot, probably through Foible.
When the play opens, Mirabell is impatiently waiting to hear that Waitwell is married to Foible. During Mirabell's card game with Fainall, it becomes clear that the relations between the two men are strained. There are hints at the fact that Fainall has been twice duped by Mirabell: Mrs. Fainall is Mirabell's former mistress, and Mrs. Marwood, Fainall's mistress, is in love with Mirabell. In the meantime, although Millamant quite clearly intends to have Mirabell, she enjoys teasing him in his state of uncertainty.
Mirabell bids fair to succeed until, unfortunately, Mrs. Marwood overhears Mrs. Fainall and Foible discussing the scheme, as well as Mirabell and Mrs. Fainall's earlier love affair. Since Mrs. Marwood also overhears insulting comments about herself, she is vengeful and informs Fainall of the plot and the fact, which he suspected before, that his wife was once Mirabell's mistress. The two conspirators now have both motive and means for revenge. In the same afternoon, Millamant accepts Mirabell's proposal and rejects Sir Wilfull Witwoud, Lady Wishfort's candidate for her hand.
Fainall now dominates the action. He unmasks Sir Rowland, the false uncle, and blackmails Lady Wishfort with the threat of her daughter's disgrace. He demands that the balance of Millamant's fortune, now forfeit, be turned over to his sole control, as well as the unspent balance of Mrs. Fainall's fortune. In addition, he wants assurance that Lady Wishfort will not marry so that Mrs. Fainall is certain to be the heir.
This move of Fainall's is now countered; Millamant says that she will marry Sir Wilfull to save her own fortune. Fainall insists that he wants control of the rest of his wife's money and immediate management of Lady Wishfort's fortune. When Mirabell brings two servants to prove that Fainall and Mrs. Marwood were themselves guilty of adultery, Fainall ignores the accusation and points out that he will still create a scandal which would blacken the name of Mrs. Fainall unless he gets the money.
At this point, Mirabell triumphantly reveals his most successful ploy. Before Mrs. Fainall married Fainall, she and Mirabell had suspected the man's character, and she had appointed her lover trustee of her fortune. Fainall is left with no claim to make because Mrs. Fainall does not control her own money. He and Mrs. Marwood leave in great anger. Sir Wilfull steps aside as Millamant's suitor; Lady Wishfort forgives the servants and consents to the match of Mirabell and Millamant.