Tracy Lord is the elder daughter of a wealthy Philadelphia Main Line socialite family. She was married to C.K. Dexter Haven, a yacht designer and member of her social set, but divorced him two years prior, because, according to her father, he does not measure up to the standards she sets for all her friends and family: He drank too much for her taste, and, according to him, as she became critical of him, he drank more. Their only interaction while married, the film's opening scene, is her breaking his golf clubs and him pushing her to the ground. Now, she is about to marry nouveau riche "man of the people" George Kittredge.

In New York, Spy magazine publisher Sidney Kidd is eager to cover the wedding, and assigns reporter Macaulay "Mike" Connor and photographer Liz Imbrie. Kidd intends to use the assistance of Dexter, who has been working for Spy in South America. Dexter tells Kidd that he will introduce them as friends of Tracy's brother Junius (a U.S. diplomat in Argentina). Tracy is not fooled, but Dexter tells her that Kidd has threatened the reputation of her family with an innuendo-laden article about her father's affair with a dancer. Tracy deeply resents her father's infidelity, which has prompted her parents to live separately. Nonetheless, to protect her family's reputation, she agrees to let Mike and Liz stay and cover her wedding.

Dexter is welcomed back with open arms by Tracy's mother Margaret and teenage sister Dinah, much to Tracy's frustration. She soon discovers that Mike has admirable qualities, and even seeks out his book of short stories in the public library. As the wedding nears, she finds herself torn between her fiancé George, Dexter, and Mike.

The night before the wedding, Tracy gets drunk for only the second time in her life, kisses Mike, and ultimately takes an innocent midnight swim with him. When George observes Mike carrying an intoxicated Tracy into the house afterward, he assumes the worst. The next day, he tells her that he was shocked and feels entitled to an explanation before going ahead with the wedding. Yet, she admits she really has none, and realizes that he does not really know her at all. He has loved her as a perfect, ideal angel, an embodiment of goodness—a virginal statue—and not as a person, so she breaks off the engagement. By now she better understands her own imperfections and her criticism of others. Tracy realizes that all the guests have arrived and are waiting for the ceremony to begin. Mike quickly volunteers to marry her, but she graciously declines because she perceives that Liz is in love with him. Then, Dexter, who clearly planned to win her back all along, offers to remarry her, and she gladly accepts.