Set in New York at the end of the 19th century, The House of Mirth describes Lily Bart’s efforts to maintain her elevated position in high society. Despite being born into an important, wealthy New York family, Lily, a beautiful twenty-nine-year-old woman, remains impoverished after her father goes bankrupt during her teenage years. Now, her only hope to stay in upper-class society is to marry a rich and powerful man, so that she can acquire the social credit and financial clout she needs to lead a luxurious life.
The novel opens on a hot afternoon in September, as Lily has just missed a train to go to Bellomont, where her friends Judy and Gus Trenor are organizing a party. Her acquaintance Lawrence Selden, who runs into her at Grand Central Station, spends a few hours with her, distracting her waiting alone in the hot city. When Lawrence asks Lily if she wants to have tea in his apartment, the young lady accepts, although she knows that it is considered risky for a young woman to spend time in a man’s apartment alone. Over tea, Lily and Selden share a moment of surprising intimacy, and Lily asks Selden if he would accept to be her friend, since she sorely lacks sincere friends around her. Though clearly taken with Lily’s grace and beauty, Selden feels that she always behaves in a calculating, premeditative way, aimed at reaching a hidden goal.
After Lily takes her leave from Selden, she runs into Simon Rosedale, a rich man who is desperate to enter the high society Lily belongs to. As Rosedale makes sly comments about the fact that Lily must have just left Selden’s apartment, Lily invents an excuse about going to her dress-maker. The excuse backfires against her, as Rosedale owns the Benedick, the apartment complex where Selden lives, and knows that there is no dress-maker there. Lily then hurries off to the train station, rejecting Rosedale’s offers to accompany her. Later, she scolds herself for potentially ruining her reputation by being seen with Selden and for rebuking Rosedale, who might one day reach a position of social prominence. She also reflects on the injustice of being an unmarried woman, since men and married women have infinitely more freedom than her, and can live independent lives without constantly worrying about preserving a reputation of chastity.
On the train to Bellomont, Lily sits next to Percy Gryce, a rich but boring man whom she tries to seduce so that he will offer to marry her. During Lily’s stay at Bellomont, her efforts to attract Gryce’s attention and respect seem to succeed until Lawrence Selden unexpectedly arrives. Intrigued by Selden’s presence, Lily wants to find out if Selden has come for her or for Bertha Dorset, with whom he previously had an affair. When Lily and Selden go on a walk, Selden admits to Lily that he only came for her, and Lily feels that she is falling in love with him. However, the two of them only talk about marrying each other in a playful tone and do not discuss the matter further.
When Lily and Selden return from their walk later than they had planned, everyone sees that they spent time together. Bertha becomes extremely jealous and, to take her revenge on Lily, decides to tell Percy Gryce that Lily’s attitude of purity and self-restraint is only an illusion. Bertha references rumors about Lily borrowing money from a man—although Lily later tells her friend Judy Trenor that the man she borrowed from is part of her family and that she repaid him—and this comment frightens Gryce, who leaves Bellomont the next morning.
Despite the severe blow of Bertha’s betrayal, which reminds Lily of her difficult financial situation, Lily succeeds in convincing Gus Trenor to invest some money for her on the stock market, since he has been benefiting from a successful “tip” given to him by Simon Rosedale. Lily believes that she can manipulate Gus to do this favor for her by behaving kindly toward him, but what she does not realize is that this conversation with Gus marks the beginning of his attraction to her. Over the course of the next few months, Lily’s financial investment seems to bear fruits, as Gus hands her thousand-dollar checks. However, in the meantime, Lily also learns that Percy Gryce is now engaged to Evie Van Osburgh, and realizes that her opportunity to secure a safe financial future for herself is once again compromised.
One day, Mrs. Haffen, the cleaning woman at the Benedick, where Selden lives, comes to Lily’s house with a packet of love letters that she believes Lily has written to Selden. However, when Lily examines the letters, she recognizes Bertha Dorset’s handwriting and decides to buy the letters to protect Selden’s reputation. Although Lily initially plans on destroying them right away, she remembers Bertha’s cruel behavior toward her and decides to keep the letters instead. The knowledge of these letters makes Lily feel like she has power over Bertha, and allows Lily to renew her friendship with her.
Lily’s social popularity reaches its climax when she performs in a party organized by Mr. and Mrs. Wellington Bry, a couple desperate for social climbing. Lily appears on stage on her own as a tableau vivant and impresses everyone with her striking beauty and grace. After the performance, she spends some time alone with Selden in the garden. When Selden declares his love for her, the two of them kiss. However, suddenly moved by the thought that they cannot be together, Lily stands up and leaves. The next day, though, she receives a note from him asking to see her and she accepts, knowing that she cannot resist her feelings for him and the sense of the power she has over him.
In the meantime, Lily goes to dinner at her friend Carry Fisher’s and then to the Trenors’ house, where she believes she is going to meet her friend Judy. Once she arrives at the Trenors’, though, Gus is there alone and reveals that he has tricked Lily in coming to his home. Ultimately, he reveals that the money Lily thought she was receiving from him as part of her original investment is in fact his own money, which he has given to her as a gift, expecting sexual favors in exchange. Horrified at this idea and scared about being trapped alone in Gus’s house, Lily insists on leaving. When Gus finally gives in and Lily steps out of the house into a cab, she believes she sees a familiar figure at the corner of the street, but does not pay attention to it.
In the meantime, after eating at his cousin Gerty Farish’s, Selden goes to Carry Fisher’s to look for Lily. When he hears that she has already left, he decides to go for a walk. Ned Van Alstyne accompanies him and, when the two men approach the Trenors’ house, they see Lily walk out and leave Gus behind. Believing that Lily is having an affair with Gus, Selden abruptly leaves, angry and hurt.
Lily, meanwhile, realizes that she cannot bear the idea of being alone in her room. Therefore, she resolves to go see Gerty Farish, an innocent woman separate from high society whom Lily realizes is her only true friend. Lily spends the night there and, the next day, returns to her aunt Mrs. Peniston’s house, where she lives. After calculating what she owes Gus Trenor, she resolves to ask her aunt for money, but Mrs. Peniston refuses to help her niece when she discovers that some of her debts are related to gambling. She scolds Lily for her rash behavior and privately condemns her for allowing rumors about accepting romantic advances from Gus Trenor and George Dorset to circulate about her.
In this dire situation, the only thing Lily can still look forward to is Selden’s visit. However, instead of Selden, Rosedale comes to Mrs. Peniston’s house and asks Lily to marry him. Lily, who feels nothing but repulsion for Rosedale, tells him she needs time to think about it. The next day, she reads in the newspaper that Selden has left for the Caribbean. Around the same time, she receives an invitation from Bertha Dorset to join her on a cruise through the Mediterranean, which Lily accepts, feeling that this is her only way of leaving her troubles behind.
Three months later, at Monte Carlo, Lily finds herself in a potentially explosive social situation, as everyone knows that her role on the trip is to distract George Dorset while his wife Bertha takes part in an adulterous relationship with the young, innocent Ned Silverton. This fragile situation collapses after George catches Bertha with Ned. To detract the attention from her own adulterous behavior, Bertha invents lies about Lily, accusing the young girl of trying to seduce her husband. As a result, Lily becomes ejected from her social circle. Weeks later, when Lily returns to New York, she realizes that everyone has turned against her and that people only believe Bertha’s version of the story, for the simple reason that Bertha is rich and powerful. In addition to this state of affairs, Lily learns that her aunt, who died suddenly, has disowned her and is leaving her just barely enough to repay her debt to Gus Trenor.
Aided only by Carry Fisher and Gerty Farish, Lily finds a series of jobs to support herself. She begins by joining the social circle of Mr. and Mrs. Gormer, although Bertha Dorset influences Mattie Gormer in rejecting Lily. Lily then serves as social secretary for a rich divorcée from the West, Norma Hatch, but soon has to leave when Norma is involved in a scandal with Freddy Van Osbourne. Rumors spread about Lily’s involvement in this scandal, further harming her reputation. Lily is then forced to accept a job at Mme. Regina’s, a hat-maker’s, although bad health and poor hat-making skills cause her to get fired. In the space of a few months, Lily thus finds herself condemned to living a working-class life, far from the comforts of her previous circle.
However, Lily remains convinced that she needs to find a way to reintegrate high society and become rich and powerful—her lifelong dream. Moved by desperation, Lily considers following Rosedale’s suggestion to use Bertha’s letters against her as a form of blackmail, so that Lily might reenter high society. Although Lily hesitates at length about whether or not she should do this, she finally experiences a moment of moral illumination and concludes that she cannot let herself behave in such an unethical way. To put an end to this temptation, she burns Bertha’s letters.
In the meantime, Lily has admitted to Lawrence Selden that his love for her has always helped her and that she has always kept the Lily Bart he knows safe in her heart. Upon returning home from Selden’s apartment, extremely weak because of constant worrying and lack of sleep, Lily runs into Nettie Struther, who Lily recognizes as a woman she had helped through Gerty’s charity. Seeing how ill Lily herself seems, Nettie takes Lily to her apartment, where she tells her all about her life, describing how, thanks to her personal strength and her husband’s faith in her, she has succeeded in escaping poverty and reaching happiness. After leaving Nettie, Lily is inspired by her impressive tale of courage and determination, and realizes that poverty is not a sign of moral degradation.
Lily then returns to her boarding-house, where she finally receives a check with the full amount of her aunt’s legacy. In order to resist the temptation of not repaying her debt to Gus Trenor, Lily writes a check to Gus immediately. Then, moved by a desperate desire to escape her dire situation and sleep, she takes a higher dose of sleeping drugs than she should. Before falling asleep, she decides that she should declare her love to Selden.
The next day, people discover that Lily has died from her drug overdose. Selden, who in the meantime had resolved to ask Lily to marry him, sees Gerty at the boarding-house and learns that Lily is dead. Overcome by sorrow, Selden begins cleaning Lily’s room and discovers Lily’s letter to Gus Trenor. At that moment, he becomes fully aware of Lily’s strong moral principles. Overwhelmed with grief, Selden finally kneels by Lily’s side. Despite knowing that they will never be able to live together and express their love for each other, he takes comfort in knowing that their love is stronger than death.