The novel opens with Newland Archer attending the opera, where New York society often gathers. Just that afternoon, May Welland has agreed to marry him. Archer notices a strange woman in May’s opera box and realizes that it’s her cousin, Ellen Olenska. It’s daring of May’s family to bring Ellen to the opera, since she has been involved in a scandal. She left her cruel husband, and it’s rumored that she had an affair with his secretary. When the opera ends, Archer tells May that he wants to announce their engagement that very night so that he can support her in any backlash that might come from Ellen’s presence.
That night, there’s a ball at the Beauforts’ house, and May tells her friends that she’s engaged to Archer. He wishes it wasn’t necessary to announce the engagement in this way, but he feels very much in love with May. The next day, Archer goes with May and her mother to visit May’s unconventional grandmother, Mrs. Mingott, who stays at home due to her obesity. When Ellen turns up there, Archer tells her about his engagement, and she invites him to come see her.
The next evening, Sillerton Jackson comes to dinner at the Archers’. He always knows all the latest gossip, and Mrs. Archer and Newland’s sister Janey want to hear about Ellen Olenska, though they avoid talking about anything truly scandalous. All this fuss about Ellen begins to make Archer question long-held values that society has imbued in him; he’s inclined to support Ellen even though society doesn’t. Archer also begins to worry that the innocence that is considered proper for unmarried women will make his marriage unsatisfying, as May has little experience with the real world.
A few days later, the Mingotts send out invitations for a dinner to welcome Ellen Olenska, but almost everyone refuses the invitation; it’s clear that they’re snubbing Ellen due to her past. Archer and his mother go visit the van der Luydens, one of the most respected couples in New York, to ask for their help (a maneuver meant to protect May’s family honor). The van der Luydens agree to invite Ellen to a dinner they’re having for a visiting duke in order to send a message that Ellen should be accepted.
At the dinner, Ellen flouts numerous social conventions. She and Archer have an intimate discussion about May and about Ellen’s desire to become completely American again after living abroad for so long. Archer finds himself flustered by her. He goes to visit her the next evening and finds that she has decorated her house in a very charming and romantic way. As Archer and Ellen talk, he realizes that she doesn’t see how precarious her position is in society. She says she wants him to help her understand New York society, but she’s already making him look at it differently.
The next day, Archer and May go for a walk and he tries to convince her to shorten their engagement, but she refuses. The following afternoon, Archer learns that Ellen Olenska went to a party at Mrs. Struthers’s house with the Duke of St. Austrey and Mr. Beaufort. Mrs. Struthers’s gatherings aren’t considered socially proper, and Mrs. Archer and the van der Luydens are upset that Ellen went. Mr. van der Luyden arrives unexpectedly. He’s been to Ellen’s house to try to help her understand her social faux pas.
Two weeks later, Mr. Letterblair, the head of the law firm where Archer works, tells him that Ellen Olenska is seeking a divorce from her husband. He wants Archer to convince her not to. He gives Archer papers relating to the case, including a letter from Ellen’s husband threatening to tell everyone that she had an affair with his secretary. Archer doesn’t want to be involved, but he goes to talk to Ellen about it. He’s angry to find Beaufort at her house, and once Beaufort leaves, Archer brings up the divorce. Ellen says she wants to be free, but her family doesn’t want her to get a divorce. Archer makes her understand that there would be negative social repercussions if she divorced, and she agrees not to.
Archer next sees Ellen at a play, where a scene of two lovers parting moves them both. May has gone to St. Augustine with her family and has asked Archer to be good to Ellen in her absence. Archer wants to see Ellen again, but he learns that Ellen has gone to visit the van der Luydens at their estate, Skuytercliff. He finds an excuse to visit her there. They end up alone in a small house on the property, and Archer imagines that she’ll declare her attraction to him. Instead, Beaufort appears, and Archer bitterly realizes that Ellen had come here to get away from Beaufort. Back at home, Archer can’t stop thinking about Ellen. When he receives a note asking him to visit her, he instead takes a boat to St. Augustine.
Archer immediately feels stabilized when he sees May. He tries to convince her to get married sooner, but she asks whether he wants to do so because he’s not confident in his love for her, and she tells him that if there’s someone else, they shouldn’t get married. Archer is amazed at her insight, feeling that this represents a new side of her that he likes, but he assures her there’s no one else.
When Archer returns to New York, he visits Mrs. Mingott, who agrees to try to convince the Wellands to move the wedding up. The next day, he goes to see Ellen and finds Medora Manson there. She wants him to help her convince Ellen to return to her husband, but he refuses.
Left alone with Ellen, Archer tells her about his conversation with May in St. Augustine. Then he admits his feelings for Ellen and kisses her. However, Ellen says that they can’t be together because Archer has taught her to see the world like a New Yorker, and New Yorkers would find their love unacceptable. Just then, a telegram arrives from May, saying that her parents have agreed to let them get married in a month.
Archer moves through his wedding in a haze, looking for Ellen, who isn’t there. He and May are supposed to spend their wedding night at his aunts’ house in Rhinebeck, but they learn that there’s a leak in the plumbing and Mr. van der Luyden has instead prepared for them the house at Skuytercliff where Archer talked with Ellen.
Archer and May go on a long trip to Europe, ending in London. They go to a dinner at the house of some friends of Mrs. Archer’s, where Archer enjoys conversation with a French tutor named M. Rivière. May, however, thinks him socially inferior.
Archer and May spend the summer in Newport, Rhode Island, along with much of New York society, and Ellen has moved to Washington, D.C. After May wins an archery contest at the Beauforts’ house, the couple goes to visit Mrs. Mingott. Unbeknownst to Archer, Ellen is also there, and he is sent to fetch her from the shore. He watches Ellen for a long time, but when she doesn’t turn to look at him, he returns to the house without speaking to her.
One afternoon, Archer finds an excuse to get away from the Wellands and he travels to Boston where he heard Ellen has gone. He finds her sitting on a bench on the Common. She has come to Boston to meet an emissary from her husband who wants her back, but she has refused to submit to his wishes. Archer convinces her to take a boat with him, and they end up in a private room at an inn. Archer realizes that she feels as much anguish about their separation as he does, but she makes him promise not to do anything that will hurt May, or else she’ll return to her husband.
Back in New York, Archer runs into M. Rivière and realizes that he’s the emissary sent by Ellen’s husband. However, M. Rivière has changed his mind upon speaking to Ellen, and he wants Archer to make sure that Ellen never returns to her husband.
At the Archers’ Thanksgiving dinner, Mrs. Archer and the Jacksons lament how much society is changing. There are rumors that Beaufort’s bank is going under, and Sillerton Jackson tells Archer that Ellen will be penniless if it does. Soon afterwards, Beaufort’s bank collapses, and Mrs. Mingott has a mild stroke. She demands that the family send for Ellen immediately.
Archer goes to meet Ellen at the station. In the carriage ride back, he insists that they need to find a way to be together, but Ellen says that they can’t. That night, Archer feels particularly oppressed by his life with May and imagines her dying. He plans to force Ellen to run away with him, but a week later, Mrs. Mingott summons him and tells him that Ellen is going to stay with her. He agrees to help convince the family of the wisdom of this plan.
Now Archer and Ellen will have the opportunity to conduct their affair in New York, but they’ll have to sneak around and lie to do so, which Archer dreads. They meet at the Metropolitan Museum of Art and agree that they don’t want to live this way. Instead, they’ll sleep together once.
The next night, Archer and May go to the opera, where everything reminds him of the first night he saw Ellen there. He decides he needs to confess everything to May and ask for his freedom, but before he can do so, May tells him that Ellen is going back to Europe the very next week. Archer is devastated.
May throws a grand farewell dinner for Ellen, which Archer drifts through, hardly aware of what’s happening. He comes to think that everyone believes he and Ellen are lovers, and they’re all conspiring to separate them without having to acknowledge what they believe. After the guests leave, Archer again tries to make a confession to May, but instead, she tells him that she’s pregnant. She told Ellen this news two weeks earlier, even though she wasn’t sure at the time that it was true.
Twenty-six years later, Archer has three children and May has died. Archer has been involved in politics and public life. His son Dallas invites him on a trip to Europe, and while staying in Paris, Dallas gets in touch with Ellen. He and Archer go to her house, but Archer sits on a bench outside rather than going in to see her.