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It is a January evening in 1870s New York City and the fashionable are attending the opera. As young Newland Archer, lawyer and man about town, gazes up at his soon-to-be fiancé, May Welland, in the Mingott-family opera box, he is disconcerted by the arrival of May's cousin, the Countess Ellen Olenska, who has left her profligate but wealthy Polish husband. To discourage gossip, Newland decides to announce his and May's engagement at the Beaufort's ball that night. All of old New York is at the ball, gossiping about the Countess. Later, when the family plans a dinner to introduce her to society, no one accepts. Without delay, the Mingott family enlist the help of ancient social sages, Henry and Louisa van der Luyden, to shore up support by inviting old New York to a dinner it cannot refuse. In this way they introduce the exotic Countess, and she finds New York society charmingly narrow and provincial compared to Paris. The next day Newland visits the Countess' small house in a Bohemian section of town. He finds her drawing room exotic and her friendship with shady financier Julius Beaufort unsettling. But he senses her loneliness and, despite some misgivings, sends her yellow roses.

The Mingotts enlist Newland's boss, Mr. Letterblair, to ask Newland to dissuade the Countess from seeking a divorce. When Newland speaks with Ellen - a passionate and exotic woman, unlike his quiet, innocent May - he finds himself falling in love with her, despite his engagement. Worried by temptation, Newland flees to Florida where May's family is vacationing and asks May to move the wedding date up. Startled, May tells him that if there is "someone else," he may have his freedom. Touched by her selflessness, Newland returns to New York. As he confesses his love to Ellen, a telegram arrives from May, saying that they can be married in a month. Newland knows his duty.

Book II of The Age of Innocence begins with May marrying Newland as New York society watches. By August, a year later, Newland and May have settled into a fashionable if boring life in New York, living in a wealthy part of town and spending summers with the rest of the rich in Newport. Ellen has moved to Washington D.C.; she returns to stay with her grandmother briefly, but later leaves to visit Boston. Still under her spell, Newland lies to his wife and follows Ellen there. Ellen promises to stay in America only if they do not hurt May with a clandestine affair. She returns to Washington. Meanwhile, Julius Beaufort's shady financial dealings catch up with him, and his wife, Regina, appeals to Ellen's grandmother for help. Mrs. Mingott suffers a stroke and sends for Ellen to nurse her; during the two-hour carriage ride with Ellen from the train station, Newland suggests they have an affair. Ellen refuses, knowing that will hurt May. He abruptly leaves the carriage and walks home. Seeing May in the library, he realizes he will dutifully stay married to her forever.

Undaunted, the next day Newland meets Ellen at the Metropolitan Museum, where she finally agrees to a future one-time affair. Elated but guilty, Newland decides to confess all to May, but she interrupts to tell him that Ellen is leaving for Europe and the Archers will give a farewell dinner for her. Shocked, Newland intends to later follow Ellen. At the dinner, however, he suddenly realizes that the entire family, including May, thinks that he and Ellen are already having an affair; giving Ellen the funds to live in Europe is the family's way of dealing with the situation. That night as he and May retire, she announces that she thought she was pregnant and told Ellen earlier, before she was really sure. But now she is sure, sealing Newland's fate forever.

The years pass. Newland is 57 and he and May have two grown children: Dallas and Mary. May has recently died of pneumonia, nursing a third child to health. Newland accompanies Dallas to Paris on a business trip, where Dallas tells Newland the Countess Ellen Olenska has invited them to dine. Newland has not seen her in 26 years. Dallas confides to his father May's deathbed confession that Newland sacrificed the one thing he loved because of duty and honor. That evening outside the Countess' apartment, Newland encourages Dallas to go up without him. In Newland's memory, their love stays forever young, perfect and unchanging over time.