Following the death of Kate’s mother, it quickly becomes clear that the money she left in her will won’t be enough to cover the family’s debts. Kate’s sister Marian is also struggling to raise two children following the death of her husband, while Kate’s father Lionel has been cast out of polite society for reasons that remain unclear and has no apparent way to make money of his own. With the family in dire financial straits, Kate feels obligated to find a way to help Lionel and Marian.
Since her mother’s death, Kate has been living with her aunt Maud. Maud is a powerful and wealthy member of London’s high society. She is convinced that with her help, coupled with Kate’s innate charm and beauty, Kate will be able to marry and wealthy and powerful man. Kate, however, is in love with Merton, a journalist of a comparatively low socioeconomic status with no economic prospects. One day, while walking down the street, Kate and Merton declare their undying love for each other. Though they consider themselves engaged at this point, they keep their engagement a secret, knowing that Kate’s family—and especially Maud—will disapprove. Maud speculates that Kate and Merton may be romantically involved, but Maud pushes Kate toward the wealthier Lord Mark as a potential husband.
At that time, Merton’s job sends him on a months-long trip to the United States to write a series of articles for his newspaper in London. On that trip, Merton briefly meets Milly, a wealthy American who is suffering from a mysterious illness. After meeting Merton, Milly sets off for Europe with her friend Mrs. Stringham in tow. The two first go to the Alps before traveling to London.
In London, Mrs. Stringham reaches out to her former classmate, Maud, whom she hasn’t spoken to in years. To her surprise, Maud writes back and invites Mrs. Stringham to dinner. Over the next week or so, Mrs. Stringham and Milly become frequent visitors at Maud’s house, and Milly in particular is well-liked in Maud’s social circle. Milly and Kate become fast friends, and Lord Mark also seems to develop a romantic interest in Milly.
Meanwhile, Merton returns to London from the United States. Upon seeing Merton, Milly realizes that she hadn’t been mistaken when she first met Merton. She finds him just as charming in London as in the United States, and she senses that she is quickly falling in love with him.
One day, Milly runs into Merton and Kate together at the National Gallery. Milly invites Kate and Merton to have lunch with her at her hotel. Milly is momentarily concerned that Kate and Merton may be romantically involved, but Milly then decides to accept the story, circulated by Maud, that Merton is infatuated with Kate even though Kate doesn’t reciprocate his feelings.
As Milly’s illness progresses, she begins seeing a doctor in London named Sir Strett. Strett tells Milly that she has no choice but to live her life to the fullest. He also advises her to leave London as soon as possible. In light of this, Mrs. Stringham decides that she will try and encourage a relationship between Merton and Milly because she wants Milly to experience love before she dies.
Kate and Maud also encourage Merton to become romantically involved with Milly. Maud hopes that if Merton and Milly are romantically involved, then Kate will turn her attention toward Lord Mark. But Kate has other ideas in mind. Without sharing the details of her plan, Kate encourages Merton to entertain Milly’s feelings for him and to continue getting closer to her. Merton is confused as to why Kate wants him to get close to Milly when he and Kate are engaged, but he goes along with the plan out of his devotion to Kate.
Milly and Mrs. Stringham then travel to Venice, and Kate, Maud, and Merton accompany them. One day, Lord Mark arrives in Venice and proposes to Milly. He explains the proposal to Milly as a kind of business transaction. Milly will be loved, Lord Mark says, and he will get her money when she dies. Milly roundly rejects Lord Mark’s proposal, and Lord Mark leaves on the first train out of town.
In Venice, Merton begins to see Milly more regularly, and the two develop a rapport. Whenever he sees Milly, though, Merton feels like he is talking with a relative, not someone he’s romantically interested in. Kate sees his lack of romantic interest and encourages Merton to try harder to encourage Milly’s feelings for him. Merton remains confused about Kate’s plan and confronts Kate. Kate ultimately reveals what she has in mind. She wants Merton to seduce Milly and marry her. Then, once Milly dies, she’ll leave her fortune to Merton. Merton and Kate will then use that money to assuage Maud’s apprehensions about their relationship and marriage, and Kate will also have money to give to her family, so Kate and Merton will be able to live happily ever after. Merton tells Kate that to go through with the plan, he’ll need “assurances” from Kate, and he pushes Kate to have sex with him. Kate ultimately agrees, and the two have sex.
Kate and Maud then leave Venice, while Merton, Milly, and Mrs. Stringham stay behind. Merton sees Milly every day until one day he arrives at Milly’s house and isn’t admitted entry. On his way back to his hotel, Merton sees Lord Mark and becomes convinced that Lord Mark is the reason that he’s not being admitted to Milly’s house. Merton spends the next few days agonizing alone in Venice. Then, Mrs. Stringham comes to his hotel to visit him. She says that Lord Mark told Milly that Merton and Kate have been engaged this whole time.
Merton visits Milly one last time. Contrary to what Merton expected, the two don’t talk about Kate. They talk as if nothing has happened, and Milly then tells Merton that he shouldn’t continue to stay in Venice on her account.
Merton returns to England. A few weeks later, he gets a letter from Milly in the mail but doesn’t open it. The next morning, he travels to Sir Strett’s house, where he learns that Milly has died. Merton takes the unopened letter to Kate and tells her to open it. Kate reads the letter and says that Milly has left Merton her entire fortune.
A month or so later, Merton gets a letter from a law firm in the United States confirming the fact that Milly has left her fortune to him. Merton forwards the letter to Kate, who comes to see him. Merton feels guilty about how he and Kate manipulated Milly, and he tells Kate that he’s not going to accept Milly’s money. He then gives Kate an ultimatum. He says that he’ll marry her tomorrow, but he doesn’t want anything to do with Milly’s money. If Kate doesn’t want to marry him, then he’ll sign over Milly’s money to her. In other words, Kate can have him or Milly’s money, but she can’t have both. Merton tells Kate that he wants them to go back to how they once were. Kate replies, “We shall never be again as we were!” and turns to leave.