Lawrence Selden, a bachelor lawyer, views the exquisitely beautiful Lily Bart, a socialite who is expected to inherit vast sums of money from her wealthy aunt, and who is further anticipated to marry a man of vast wealth. Lily is twenty-nine years old and worries that her physical beauty is fading, which means that her chances for marrying an eligible wealthy man are also becoming slimmer. An orphan, Lily lives with her wealthy aunt, Mrs. Peniston, a miserly woman who refuses to give her niece an allowance.
Selden and Lily retire to Selden's apartment for tea and discuss the relative drawbacks to womanhood. Lily reveals to Selden that her manner has served to put off a potential mother-in-law, who has since sent Lily's prospective fiancé to India. Both Selden and Lily agree that the social functions they attend are boring affairs. Upon leaving Selden's apartment, Lily meets Simon Rosedale, a bachelor whose wealth continues to grow staggeringly, but whose presumptuous demeanor Lily finds repulsive.
Lily attends a weekend party at the country home of Judy and Gus Trenor, where she attempts to cement an engagement to the wealthy — and boring — young bachelor Percy Gryce. Lily turns her attentions to Selden, but finds that she has competition from Bertha Dorset, a married guest who has designs on Selden. Lily and Selden have a conversation in which it is decided that he is not wealthy enough to marry Lily. In the meantime, a bitter Bertha sabotages Lily's chances with Gryce by shocking him with the knowledge that Lily plays cards for money. On a ride to town, Trenor tells Lily that he will invest money for her with no financial risk.
Lily is approached by a cleaning woman who has recently been fired from the building where Selden lives. Believing that Lily is the woman responsible for writing love letters to Selden, the woman wishes to sell the letters to her. Lily purchases the letters and discovers that they have been written by Bertha. In the meantime, Trenor's investments on Lily's behalf are paying off, and Rosedale continues trying to court Lily. Grace Stepney, Lily's cousin, is embittered by Lily's removal of Grace from a dinner party attendance list, and proceeds to tell Mrs. Peniston that Lily has been gambling and spending extravagantly. She also tells Mrs. Peniston that there are rumors that Lily is receiving money from Trenor, in effect making her a "kept woman."
Judy has also heard of the financial arrangement between her husband and Lily, and disassociates herself from Lily. Lily attends a social function at the home of Mr. and Mrs. Wellington Bry, two nouveau riche socialites. She poses in a tableau vivant, and her beauty is made apparent to all who attend. Selden professes his love for Lily, but she rebuffs him. The following evening, Lily meets with Trenor. Trenor feels that Lily owes him her physical affections for his financial assistance. Feeling violated, she leaves, and spends the night with Selden's cousin, the virtuous and physically undistinguished Gerty Farish, a kindly and charitable woman who has recently discovered that she is in love with Selden. The following morning, Lily determines that she will repay Trenor. She rebuffs another proposal from Rosedale and waits for the arrival of Selden. She discovers that Selden has gone to Europe and receives an invitation to join the Dorsets on a Mediterranean cruise.
During the cruise, Lily is a success with the European crowd, much to the dismay of Bertha. Bertha wishes to employ Lily as a means to distract her husband, George Dorset, while she engages in a flirtation with Ned Silverton. When Silverton and Bertha miss a train back to where the Dorsets' yacht is docked, Dorset and Lily return without them. Bertha returns to the yacht at seven o'clock the following morning. In order to excuse her absence, she insinuates that Lily and Dorset parted without her and Silverton in order to indulge in their own flirtation. Lily is told she must not return to the yacht.
The European stories regarding Lily's activities reach America and Mrs. Peniston, who dies leaving only $10,000 to Lily, a legacy that will take some time before being fulfilled. Rejected by her friends for the perceived indiscretion with Dorset, Lily finds work with the Gormers. Bertha, however, befriends Mattie Gormer, and Lily is soon out of work. She then finds work as a secretary to Mrs. Norma Hatch, a divorcée and recent addition to New York's wealthy elite. Norma has designs to marry Freddy Van Osburgh, a member of Lily's previous social group. Fearing charges of impropriety, Lily quits her job. She works, ineptly, at a millinery shop, and is laid off. Her mounting bills and increasing sleeplessness cause her to rely on chloral (chlorinated ethyl alcohol) in order to rest and forget her financial worries.
Lily resumes her contact with Rosedale, who offers to marry her if she uses Bertha's letters to Selden to even the score with Bertha. After some consideration, Lily begins to take the letters to Bertha, but she has an attack of conscience and visits Selden instead. She deposits the letters into the flames of a fire in his apartment. Neither Selden nor Lily can profess their love, and she leaves his apartment. The following morning, Selden realizes that he still loves Lily. He goes to her boardinghouse to tell her, but is greeted by Gerty, who tells her cousin that Lily has overdosed on chloral the night before and is now dead.