Newland Archer finds himself in a state of elation following his engagement to the stunning young May Welland. His happiness is soon disrupted, however, by the arrival of May's cousin, Countess Ellen Olenska, freshly returned to America after leaving her unfaithful husband. With her daring attire, relaxed demeanor, and scandalous rumors surrounding her, the Countess ignites shockwaves among New York's high society. Yet, due to her influential family under the matriarch, Mrs. Manson Mingott, May and Archer feel obliged to keep her within their circles.

As he gets to know Ellen better, Archer grows fond of her unorthodox perceptions of their social world. Simultaneously, his enthusiasm for his bride-to-be, May, dwindles as he perceives her as a reflection of her societal upbringing: courteous, naive, and lacking personal conviction and identity. When the Countess announces her plans to divorce her husband, Archer, while supportive of her pursuit for independence, feels obligated to convince her to stay married for the sake of her family's reputation. During a retreat near Hudson, Archer confesses to himself his love for Ellen, leading him to suddenly leave for Florida where he reunites with May and her family. Once there, he tries to hasten their wedding plans. May grows suspicious, questioning if his rush stems from doubts about their union. In an effort to pacify her, Archer insists he is indeed in love with her. Upon returning to New York, he admits his love to Ellen, only to receive a message from May, stating that their wedding has been expedited.

Following their wedding and a honeymoon in Europe, Archer and May begin their life as a married couple in New York. Over the years, his memories of Ellen become a distant, yearning image. However, a chance reunion with her in Newport rekindles his feelings. She promises to stay in America as long as their love remains unexpressed. Back in New York, Archer discovers that Count Olenski wants Ellen back, but she refuses. After her grandmother suffers a stroke, Ellen returns to New York to look after her. She and Archer plan to surrender to their passion, but suddenly, Ellen decides to return to Europe. May, who has just announced her pregnancy, throws a farewell party for Ellen. After the guests depart, she reveals to Archer that she had shared the news with Ellen a fortnight ago.

Fast forward twenty-five years, Archer and May have had three children, and May has passed away from pneumonia. Archer's son, convincing his father to visit France, arranges a meeting with Countess Olenska at her Parisian residence. But Archer, in the end, sends his son to see her alone, deciding to cherish the memories of his past instead.