Undine Spragg, who moves to New York from Apex City with her parents, is in the city for two years without being accepted into society. Her opportunity comes at last when she is invited to a dinner given by Laura Fairford, whose brother, Ralph Marvell, takes an interest in her. Although his family is socially prominent, Ralph has little money. He is an independent thinker who dislikes the superficiality of such important people as Peter Van Degen, the wealthy husband of Ralph’s cousin, Clare Dagonet, with whom Ralph was once in love.

About two months after their meeting, Undine and Ralph become engaged. One night, they go to see a play, where Undine is shocked to find herself sitting next to Elmer Moffatt, someone who knows about her past. She promises to meet him privately in Central Park the next day. When they meet, Moffatt, a blunt, vulgar man, tells Undine that she must help him in his business deals after she marries Ralph. Moffatt also goes to see Undine’s father and, threatening to reveal Undine’s past if Mr. Spragg refuses, asks him to join in a business deal.

Mr. Spragg is fortunate in his business deal with Moffatt and is able to give Undine a big wedding. After Ralph and Undine are married, Ralph gradually realizes that Undine cares less for him than for the social world. He also becomes aware of Undine’s ruthless desire for money. Her unhappiness and resentment increase when she learns that she is pregnant.

During the next several years, Moffatt becomes a significant financial figure in New York. Ralph, in an attempt to support Undine’s extravagance, goes to work in a business to which he is ill-suited. Undine, meanwhile, keeps up a busy schedule of social engagements. She also accepts some expensive gifts from Peter Van Degen, who is interested in her, before Peter leaves to spend the season in Europe. One day, Undine sees Moffatt, who comes to propose a disreputable business deal to Ralph; the deal succeeds and Undine goes to Paris to meet Peter, where soon she spends all the money. She meets the Comte Raymond de Chelles, a French aristocrat, whom she thinks of marrying until Peter tells her that if she stays with him, he will give her everything she wants. At this point, Undine receives a telegram announcing that Ralph is critically ill with pneumonia and asking her to return to New York immediately. Undine decides to stay in Paris.

Ralph recovers. After her uncontested divorce from Ralph, Undine lives with Peter for two months. When he learns that Undine did not go to Ralph while he was ill, Peter is disillusioned and leaves her without getting the promised divorce from his wife Clare. Ralph, meanwhile, returns to the Dagonet household with his son, Paul, for whose sake he begins to work hard at the office. He also resumes work on a novel. Then he learns that Undine is engaged to Comte Raymond de Chelles and badly needs money to have her marriage to Ralph annulled by the church. Undine agrees to waive her rights to her son if Ralph will send her one hundred thousand dollars to pay for her annulment. Ralph borrows half of the needed sum and goes to Moffatt to make another business deal. As Undine’s deadline approaches, with the deal not yet concluded, Ralph consults Moffatt, who tells him that the matter is going more slowly than expected and that it will take a year to materialize. Moffatt tells Ralph that he himself was once married to Undine, back in Apex City, but that Undine’s parents forced the young couple to get a divorce. After hearing this story, Ralph goes home and commits suicide.

Undine, now in possession of her son, marries Raymond de Chelles. She is very happy in Paris, even though Raymond is strict about her social life. After three months, they move to the family estate at Saint Desert to live quietly and modestly. When Raymond begins to ignore her, Undine becomes bored and resentful of her husband’s family for not making allowances for her extravagances, which continue unabated.

One day, she invites a dealer from Paris to appraise some of the priceless Chelles tapestries. When the dealer arrives, the prospective American buyer with him turned out to be Moffatt, now one of the richest men in New York. Over the next several weeks, Undine sees a great deal of her first husband. When the time comes for Moffatt to return to New York, Undine invites him to have an affair with her. Moffatt tells her that he wants marriage or nothing.

Undine goes to Reno, Nevada, where she divorces Raymond and marries Moffatt that same day. Moffatt gives Undine everything she wants, but she realizes that in many personal ways he compares unfavorably with her other husbands. The Moffatts settle in a mansion in Paris to satisfy Undine’s social ambitions and her husband’s taste for worldly display. When Undine learns that an old society acquaintance, Jim Driscoll, is appointed ambassador to England, she decides that she would like to be the wife of an ambassador. Moffatt tells her bluntly that that is the one thing she can never have because she is a divorced woman. Still dissatisfied, Undine is certain that the one thing she is destined to be is an ambassador’s wife.