The film begins on the weekend of Philadelphia socialite Tracy Lord's second marriage, to a young self-made man, George Kittredge. Having recently divorced her husband of two years, the old-money C.K. Dexter Haven, on the basis that he drinks too much and was a bad partner, Tracy is ready to start anew with a more reliable and steadfast spouse. Making matters all the more complicated is the fact that Tracy's father, Seth Lord, is away in New York philandering with a young chorus girl and will likely not even be present at the wedding.
Meanwhile, Sidney Kidd, the editor of a tabloid Spy Magazine, has his sights set on having reporter Mike Connor and photographer Liz Imbrie cover the wedding. Enlisting the help of Tracy's ex-husband Dexter (who has been working for the magazine in South America), Kidd has plans for his reporters to infiltrate the wedding undercover and get a juicy story about the wealthy debutante. Dexter invites Mike and Liz into the Lords' home, introducing them as friends of Tracy's brother. However, Tracy doesn't believe Dexter, and manages to get him to spill the truth, that they are reporters for Spy Magazine. When Tracy orders Dexter to get them off the premises, Dexter reveals that Sidney Kidd has a salacious story that he wants to break about her father's affair in New York, and that letting Mike and Liz cover her wedding is the only way to prevent the exposure of her father's indiscretion. Left with no other choice, Tracy agrees.
Tracy's sister, Dinah, and her mother both adore Dexter and welcome him back into the family home even as Tracy becomes more annoyed with his presence. After making a comical performance of wealthy society life for the benefit of the reporters, the Lords reveal to Mike and Liz that they know their true identities. As Tracy gets to know Mike Connor, an aspiring and struggling short story writer, she finds herself more and more charmed. His irreverent attitude towards her society upbringing is intriguing and he finds her more astute and fascinating than he had previously expected. Though she spars with Dexter, Tracy remembers her attraction to her debonair ex-husband, and begins to have unspoken doubts about her wedding to George.
At a party the night before her wedding, Tracy drinks too much. Drunkenly, Mike tells Dexter a number of incriminating secrets about Sidney Kidd, and the two set about writing up an article to expose him. When Mike arrives back at the Lord estate, he and Tracy drink champagne and argue, before abruptly embracing, sharing a kiss, and going off to swim together. When George Kittredge comes to check on Tracy, he sees Mike carrying his bride back to the house and imagines they are having an affair. The following day, George confronts Tracy about her indiscretion, and his angry protestations and territorial brutishness confirms for Tracy that they are not a match. She rejects him and calls off the engagement.
Without a groom, Tracy runs to her own wedding ceremony and sets about making a speech about the unexpected change of plans. Mike offers to marry Tracy, but she suggests that he is better suited for Liz. Seeing his opportunity, Dexter proposes to Tracy and they get married then and there. The film ends with Sidney Kidd snapping a photo of the couple for Spy Magazine from the congregation.