Warning: wikipedia contains spoilers
The plot concerns Pinchwife and Margery, a married couple who come up to London for the wedding of Pinchwife's sister, Alithea. The groom, Sparkish, is attracted to Alithea only by the large dowry Pinchwife is paying. Margery, a country girl, is much younger than her husband, Pinchwife, and is seduced by his friend, Horner. Unknown to her, Horner has other lovers, and the women are obliged to unite to prevent the discovery of their unfaithfulness. Meanwhile, the hero, Harcourt, falls in love with Alithea and rescues her from marriage to Sparkish.
Comments from the 1911 Britannica:
So strong, indeed, is the hand that could draw such a character as Marjory Pinchwife (the undoubted original not only of Congreve's Miss Prue but of Vanbrugh's Hoyden), such a character as Sparkish (the undoubted original of Congreve's Tattle), such a character as Horner (the undoubted original of all those cool impudent rakes with whom our stage has since been familiar), that Wycherley is certainly entitled to a place alongside Congreve and Vanbrugh. And, indeed, if priority of date is to have its fair and full weight, it seems difficult to challenge Professor Spalding's dictum that Wycherley is "the most vigorous of the set."
In order to do justice to the life and brilliance of The Country Wife we have only to compare it with The Country Girl, afterwards made famous by the acting of Mrs Jordan, that bowdlerized form of The Country Wife in which Garrick, with an object more praiseworthy than his success, endeavoured to free it of its load of unparalleled licentiousness by disturbing and sweetening the motive--even as Voltaire afterwards (with an object also more praiseworthy than his success) endeavoured to disturb and sweeten the motive of The Plain Dealer in La Prude. While the two Bowdlerized forms of Garrick and Voltaire are as dull as the Aesop of Boursault, the texture of Wycherley's scandalous dialogue would seem to scintillate with the changing hues of shot silk, or of the neck of a pigeon or of a shaken prism, were it not that the many-coloured lights rather suggest the miasmatic radiance of a foul ditch shimmering in the sun.