The Spraggs, a family of newly wealthy midwesterners from Apex, arrive in New York City to advantageously marry off their beautiful, ambitious, and temperamental daughter Undine. Attracted to glamour and extravagance Undine has a hard time making inroads into the high status old money social circles she wishes to enter. Her beauty catches the attention of several men who offer her a tantalizing glimpse into their world. Ralph Marvell, who is descended from the Dagonets, an old money family, becomes attracted to Undine. Convinced that Undine is a simple and plain spoken girl who would be ruined by her elevation in society, he resolves to quickly woo and marry her. However, Marvell is an unsuccessful lawyer and a would-be poet, and his family no longer has great reserves of cash. Before the marriage, his grandfather informs Mr. Spragg that Spragg will have to financially support the couple. Mr. Spragg asks Undine to end her engagement but Undine, now aware that the Marvells and the rest of their social circle highly value sexual purity and frown upon broken engagements and divorce, refuses to leave Ralph, and her father relents to the marriage. Shortly before her wedding, Undine encounters an acquaintance from Apex named Elmer Moffatt. Undine begs him not to make their acquaintance known as it could compromise her relationship with Ralph. Elmer agrees, but later, he approaches Mr. Spragg and leverages the fact that the Spraggs are embarrassed to know him to coerce Mr. Spragg into a business deal that profits them both greatly.
Ralph and Undine marry quickly and honeymoon in Europe. Although Ralph dotes on Undine, their relationship quickly comes into conflict. Neither one of them enjoys the other's activities and his attempts to be a moderating influence on her extravagance are ignored. Worst of all his finances do not permit the lifestyle Undine desires. After Undine's father is unable to send them money Undine forces Ralph to extract money from his sister and her husband which Ralph resents. At the end of their honeymoon Undine discovers that she is pregnant. She is horrified by the news and Ralph realizes he is as well.
Four years later, Undine misses her son Paul's birthday, causing Ralph to realize that he is no longer in love with her. The couple are deeply in debt due to Undine. She resents Ralph for his lack of funds, and he resents her for forcing him to work. Unable to cover her bills, Undine accepts a loan from Peter Van Degen, the nouveau riche husband of Ralph's cousin Clare. Peter is a known philanderer, and Undine flirts with him, hoping that an affair will extract more financial aid. However when Peter abruptly leaves for France, Undine realizes that she would be happier if she divorced Ralph and married Peter. Undine fakes an illness so that Ralph will send her to France to recover. While there, she convinces Peter to leave Clare and marry her.
After Undine and Ralph's divorce, Peter reunites with Clare and refuses to see Undine. She later learns from a friend that, while Clare never would have agreed to a divorce, the reason that Peter dropped Undine was that he discovered Ralph was deeply ill and was pleading for her to come home. Peter's fear that Undine would do the same to him led him to end their relationship.
Her circumstances and social status greatly reduced, Undine returns to Paris, where she is fortunate enough to meet Raymond de Chelles, a French count, who falls in love with Undine. The de Chelles are Catholics and frown upon Undine's marriage and divorce. Undine discovers that an annulment is possible but does not have the financial means to procure one. She runs into Elmer Moffat who suggests that she use her legal hold on her son Paul to extract the money from Ralph.
Ralph, whose family has been raising Paul since his divorce, is shocked to discover that Undine now wants him to live in France with her. His cousin Clare points out that, rather than legally fight for custody, he should offer Undine a large amount of money to keep Paul. Ralph borrows from his inheritance and goes to Elmer Moffat in the hope of doubling his money. However the funds do not come through in time, and at the same time, Elmer informs him that he and Undine were married years ago in Apex. In shock and grief, Ralph commits suicide. His son is his sole heir, and when the funds Ralph invested finally do come through, they are controlled entirely through Undine who, through Ralph's death, is able to marry Raymond.
Undine is soon dissatisfied with Raymond, too. The de Chelles are hidebound aristocrats, their wealth tied in land and art and antiques that they will not consider selling, and Undine cannot adjust to the staid customs of upper-class French society. She also resents having to spend most of her time in the country because her husband cannot pay for expensive stays, entertainment, and shopping trips in Paris.
Undine at last runs into Elmer Moffat, now extremely wealthy and successful. They renew their acquaintance, and Undine realizes that he is the only man she really loved. She suggests that they begin an affair, which will be tolerated by her husband and his family as long as they are discreet. To her surprise, Elmer refuses and insists that he will only renew their relationship if she divorces Raymond and marries him.
Now, married to the crass midwestern businessman who was best suited to her in the first place, Undine finally has everything she ever desired. Still, it is clear that she wants even more: In the last paragraph of the novel, she imagines what it would be like to be an ambassador's wife, a position closed to her owing to her divorces.
