New York high society of the 1870’s. Newland Archer, a successful young lawyer, is engaged to the lovely but conventional May Welland; their marriage will unite two of New York’s most respected families. The Countess Ellen Olenska, May’s cousin, returns to her native New York from Europe in an aura of scandal, having abandoned her dissolute husband, the Polish Count Olenski. Archer tries to persuade May, her mother, and her grandmother, Mrs. Catherine Mingott, to advance the date of his wedding, but he also finds himself drawn to Ellen, and takes pity on her and her status as an outcast in New York. Ellen further alienates herself by making clear her intention to sue her husband for divorce, which would constitute a social nightmare for the Archers and Wellands. Mrs. Mingott prevails upon Archer to force Ellen to drop the case, which he succeeds in doing. Meanwhile, May suspects that Archer might have feelings for someone else (though she does not guess the woman is Ellen) and offers Archer a chance to end their engagement. He insists he loves her only. By the time Ellen and Archer confess their love for each other, May announces that her and Archer’s mothers have agreed to a wedding before Easter.
A few months after his wedding, Archer realizes that his marriage is fast becoming a prison for him. Mrs. Mingott tells him that Ellen, who has moved from New York to Washington, has heard from her husband: he wants her back, on her own terms. Archer goes to see Ellen and finds out she has refused the Count’s offer. They reaffirm their love for each other, but Ellen insists that they cannot betray the family, and therefore they must remain apart—if Archer tries to have her, she will have no choice but to return to Europe. Archer retreats more and more into his own world with its visions of Ellen, as the Archers and Mingotts begin to question his loyalty to his wife and the family. A financial scandal brought on by the shady dealings of the banker Julius Beaufort, who pleads with Mrs. Mingott for monetary assistance, causes the old lady to have a stroke, after which she asks for Ellen to come take care of her. Ellen returns to New York, and Archer has a clandestine meeting with her, begging her to run away with him. She finally consents to one night of passion with him, after which she will return to Europe. Convinced that once her has her she will not be able to leave him, Archer agrees to the plan. The day before he and Ellen have their tryst, May tells him that Mrs. Mingott has made Ellen financially independent of the Count, and she is returning to Europe in a week. Archer is dumbfounded. During his and May’s farewell dinner for Ellen, Archer realizes that all of New York believes him and Ellen to be lovers, and they have now rallied around May in order to exile Ellen to Europe. Later that evening, Archer is on the verge of asking May for the freedom she once offered him before their wedding so he can go be with Ellen. May instead tells him what she had already told Ellen two weeks ago: she is pregnant.
Twenty-five years later, Archer, now fifty-seven, and his twenty-five-year-old son, Charles, are in Paris, on their way to visit the Countess Olenska. Charles reveals that he knew about Ellen and his father—when Archer asks how he could know such a thing, he explains that the day before May died ten years ago, she said she knew her children would always be safe with their father because once, when she asked him, he gave up the thing he wanted most. Arriving at Ellen’s address, Archer sends his son ahead to see her without him, feeling that Ellen is more real to him if he does not see her as she is now. He turns and walks back to his hotel.