Meet Miss Lily Bart. She's 29, beautiful, and poised to marry a rich, boring bachelor in New York in the late 1800s. Lily's main candidate is Percy Gryce, millionaire and bore-extraordinaire. While staying with a rich couple, Judy and Gus Trenor, Lily gets a chance to spend some time with Percy. But she is distracted by Lawrence Selden, a young lawyer who doesn't have a lot of money, but who is clearly Lily's intellectual and emotional soul mate.
To make matters more complicated, Bertha and George Dorset – another wealthy couple – are also in the picture. Bertha once had an affair with Selden, and she's jealous of Lily for captivating his attention. When Lily flirts with Selden, Bertha tells Percy, who runs away.
Lily is determined to get her rich potential-husband back, but before she can, he gets engaged to another woman. Lily, whose parents are both dead, lives with a wealthy aunt, but has no money. She needs cash to keep her going until she marries, so she asks Gus Trenor (Judy's husband) to speculate on the stock market for her. Gus agrees and the money starts flowing.
Meanwhile, Lily wards off the attentions of a social climber named Simon Rosedale. Rosedale seemingly has more money than the rest of the world put together, and he's trying to use it to break into the social circles that Lily frequents. She's not interested and snubs him.
And now for an interesting plot twist. A charwoman (a woman who cleans houses) who once saw Lily with Lawrence Selden drops by to see her with a series of love letters she stole from Selden's garbage. She thinks the letters belong to Lily. Looking them over, Lily realizes they were written from Bertha Dorset to Selden. She now has proof of the alleged affair. Lily purchases the letters and puts them away for safe-keeping.
One night, Lily gets an invitation to visit Judy Trenor at her home. She goes, only to find that Gus Trenor is there by himself and has now compromised Lily by getting her alone with him in the late hours of the night. Trenor explains that he's been straight-up giving Lily all the money she thought she had earned on the stock market, and now she owes him (meaning, he wants her to sleep with him). Horrified and ashamed at her position, Lily flees the house.
Meanwhile, Lawrence Selden has decided he's in love with Lily and wants to marry her. While he's working on the wording for his proposal, he sees Lily run out of Gus Trenor's house in the late hours of the night. He's super angry and high-tails it for Europe. The next day, Simon Rosedale proposes to Lily, and she turns him down. Soon after, Lily receives an invitation from Bertha Dorset to join her and her husband George on a cruise in the Mediterranean.
So, Lily heads for Europe. It's clear to everyone that Bertha invited her along to keep her husband, George, distracted while she carries on an affair with a young man named Ned Silverton. Lily is caught in the crossfire when scandal arises; Bertha is out all night with Ned, alone. In order to deflect the attention from herself, Bertha accuses Lily of having an affair with George and publicly disgraces her. Selden, who shows up in the Mediterranean just in time, tries to help Lily, but to no avail.
Lily heads back to America when her Aunt Julia (the rich aunt she used to live with) dies. Lily is relieved to finally stop worrying about her finances, since her aunt had about $400,000 to her name. Unfortunately, Aunt Julia heard about the scandal in Europe and essentially wrote Lily out of the will (she only left Lily ten thousand dollars, and it will take a year for her to see any of that money).
Distraught, Lily turns to two friends, Gerty Farish (Selden's cousin) and divorcee Carry Fisher. Both women try to help her through the difficulties. Mrs. Fisher sets Lily up as a social networker, first for a woman just below the elite on the social ladder, and then for someone lower down when that doesn't work out (thanks to Bertha Dorset's sabotage).
In the meantime, Lily has been trying to win back the affections of Simon Rosedale. Rosedale admits that he is in love with Lily, but he wants a wife who will root him firmly among society's elite, and Lily is no longer in a position to do so. He says that if she reconciles with Bertha Dorset, he will marry her. He suggests that she uses those blackmail letters she bought from the charwoman…
Lily doesn't want to break up the Dorset marriage, despite several pleas from George Dorset to help him do so. Instead, she sinks further and further into poverty, finally taking a job as a milliner (hat maker) but getting fired for being incompetent. She starts taking a prescription drug called chloral to help her sleep at night. She waits desperately for her ten thousand dollar inheritance, but she knows that she wants to pay Gus Trenor back for the "speculation" he did for her, so that she can get out of his debt. (She owes him about nine thousand dollars.)
On the brink of misery, Lily decides to take Rosedale up on his still-standing offer and blackmail Bertha Dorset with the letters. On the way to the Dorsets', though, she has a moral awakening and instead stops by Selden's place. She thanks him for always having faith in her as a good person, and burns Bertha's letters when he has his back turned.
Lily returns home for the night to discover that her inheritance check has finally come. She writes out a check to Gus Trenor for the nine thousand she owes him, puts everything in order, and takes her chloral – more than the maximum allowed dosage – to help her sleep. Whether or not Lily intentionally commits suicide or accidentally overdoses is unclear.
The next morning, Selden wakes up and decides that he's in love with Lily again. He rushes over to her apartment to propose, only to find that she's dead of an overdose. He goes through her personal things, finds the inheritance money and the check to Gus Trenor, and pieces together the whole story. He now loves the Lily more than ever.