Lily Bart is an attractive woman with some important social and family ties, but at the age of 29, she is still not married. Since the death of her mother, who had an intense hatred for "dinginess," Lily began to live with her aunt, Mrs. Peniston. However, Lily spends much of her time staying at the Bellomont, the out-of-town estate of the wealthy and well-establish Gus and Judy Trenor. At the Bellomont, Judy regularly throws extravagant parties that are attended by most of the New York upper-crust. They play bridge for money, which is problematic for Lily because she has a gambling addiction and cannot stop gambling, even though it ruins her financially.
Lily has two main goals in the book: marriage and wealth. It is her hope to marry a rich man, thereby securing her place in society, but due to her own indecision, she passes up numerous chances, always thinking she can do better. Unfortunately, Lily's true love, Lawrence Selden, does not have enough money for her to marry him.
Lily hears about the stock market at the Bellomont and decides that she would like to get involved in investment. She asks Gus Trenor to invest her small sum of money for her, and he readily assents because he is secretly attracted to Lily and wants her to spend time with him. The investments pay off, and as Lily begins to make money from Wall Street, she begins to spend lavishly. Later, to her horror, Trenor tries to proposition her, and she learns that he has not been investing her money—of which there is none—but rather his own; he has been giving her his profits. He says that she may pay him back by spending time with him, but Lily withdraws quickly from his presence and resolves that she will somehow pay him back, although she does not know how.
Lily takes a sudden vacation to the Mediterranean with George and Bertha Dorset and the young Ned Silverton, but she soon learns that she is being brought along to distract George while Bertha has an affair with Ned. When Lily begins to associate with European royalty, Bertha becomes jealous and kicks her off the cruise yacht, starting a nasty rumor that Lily and George are having an affair, which leads to Lily's expulsion from society. Lily returns to America to learn that her aunt has died, leaving her with only $10,000, which is just enough to pay off her debt to Trenor, although Lily cannot have the money until one year has elapsed. In the meantime, Lily tries desperately to rebuild her reputation in society, but fails and moves in with Gerty Farish, Selden's cousin. She takes up jobs as a secretary and milliner (hat maker) before finally moving into a boarding house and leading a meaningless life. She begins to take sleeping pills to help her get away from her fear and loneliness until the day when her check from her aunt's estate finally arrives. She pays off all her debts with the money, then overdoses (perhaps intentionally) on sleeping pills. The next day, Selden shows up at her apartment with the intention of proposing marriage to her, only to find her dead, with all her debts paid off.