Bored with her companions, popular stage star Anna Kalman returns early from Europe to her fashionable London apartment, surprising her housekeeper and chauffeur, Doris and Carl Banks. Anna, who is single and nearing middle-age, confides to her sister, Margaret Munson, that her recent, handsome paramour could not hold a conversation, but Margaret tells her not to expect so much from a man and consoles her that “someone will turn up.” She invites Anna to join her and her husband Alfred at a banquet hosted by the Foreign Office, in which Alfred holds an important position. Anna declines, being uninterested in listening to the banquet speaker discuss “hard currency,” and continues preparing for a quiet evening by removing her makeup with cold cream. Unknown to Anna, Alfred has invited Philip Adams, an American banker-diplomat from Paris who is to address the banquet, to use the apartment to change his clothes. When Philip arrives, Anna is surprised by the unexpected and handsome guest, and he is equally impressed with her, despite the cold cream on her face. Noting Anna’s interest, Margaret asks her where “Mrs. Adams can dress," as a way to determine if Philip is married and Philip says, "There is no Mrs. Adams." Philip convinces Anna to attend the banquet and while the others are changing clothes, Alfred tells Margaret that the Foreign Office is trying to woo Philip to accept a NATO post in Paris. At the end of the evening, while waiting in Anna’s apartment for his train back to Paris, Philip tells her that he is ambivalent about the NATO job. Because their mutual attraction seems obvious, Anna invites Philip to the ballet on Saturday, to which he responds that he is married. To clear up the misconception he caused earlier, Philip explains that he meant his wife is not “here tonight” and says that he is separated, but cannot divorce. Having clarified his situation, Philip contends that there are chivalrous “rules” of behavior for men concerning women. At first, Anna is resigned to never see Philip again, but as he is leaving the building, she has the concierge intercept him and again invites him to the ballet. On Saturday, he arrives at her apartment, preceded by dozens of yellow roses he has sent. As they walk to the exclusive Players’ Club for dinner, Anna is stopped by fans seeking autographs. At dinner, they are so immersed in their conversation that they are late for the ballet, and then give up their seats to a young couple and return to the club to talk. Reluctant to end the evening, they walk along the Thames, while she tacitly decides whether to continue the relationship. When they reach her apartment, Anna invites Philip up for a drink. In the morning, when Philip phones Anna from his hotel room bed, she invites him to her apartment for breakfast. Over the meal, Philip tells Anna that he has decided to take the NATO job. In the following months, their relationship flourishes, as Philip calls Anna every night from Paris and visits on weekends, finally taking a second apartment in her building. Although they conduct their relationship discreetly to protect Anna’s reputation with her public, Margaret warns Anna that NATO, through Scotland Yard, keeps track of Philip’s activities and is aware of their affair. Margaret also tells Anna that Philip is married and is surprised that Anna already knows, yet still hopes to marry him in the future. Anna begins work on a new play, which eventually opens to great acclaim. Her relationship with Philip grows stronger, and they spend Christmas together. When she learns that Philip has bought the Sea-Witch , the yacht she tried to charter for her birthday, she suggests that he spends money on her because of his guilty conscience. Despite the fact that Philip is married, Anna is content with her life. Doris is pleased for Anna’s happiness, but Carl pessimistically predicts that “it can’t go on with him married.” One day, Philip returns from Paris with news that NATO is relocating him for five months to a New York post. Unhappy, Anna suggests he get a divorce and marry her, but then, horrified by her behavior, apologizes profusely. Because his ship leaves the next day and he will miss her birthday, Philip asks her to drink a toast at the first stroke of Big Ben at midnight, and promises that he will do the same from his ship at sea. That day, while playing snooker with Alfred, Philip asks for permission to fly to New York instead of sail, so that he can spend three extra days with Anna and surprise her by walking into her apartment on her birthday at midnight. Alfred then asks why Philip pretends to be a married man when he is not. Philip, who is surprised that Alfred knows the truth, explains his position: When a man meets and is attracted to a woman, he courts her and, if they are old enough, she “favors” him. She wants to get married, and, if he is “not the marrying kind,” she is destined for disappointment. Women, Philip says, never believe a man who says he will never marry. Therefore, because he is uninterested in matrimony, he tells a woman early in their relationship, before she gives her “favors,” that he is already married, so that she will never expect more from him. Philip considers his action a kind of chivalry, and adds that he loves Anna in a way he has never loved before. Meanwhile, Anna decides to leave her play and fly to New York, where she can surprise Philip. When she tells Alfred, he feels compelled to reveal Philip’s plans to be with her on her birthday. Anna cries in delight, gushing that Philip is good and kind until Margaret blurts out that he is not. By looking at Alfred’s confidential documents from Scotland Yard behind his back, Margaret has learned that Philip has been lying about his marital status. Anna, humiliated to think how she begged his forgiveness for asking him to marry her, becomes angry, exclaiming, “How dare he make love to me and not be a married man!” Despite her anger, Anna decides to pretend she knows nothing and to attend a party with him. At the event, Anna and Philip dance a complicated Scottish reel, during which Philip, pleased with the way his plans are commencing, gleefully covers his lack of dancing skills with playfulness, oblivious to Anna’s growing resentment. During the event, one of Anna’s former paramours, David, sends her a single red rose as a token of friendship, which inspires her to plan revenge. To make Philip jealous, she secretly invites David to come to her apartment on her birthday at 11:30. Returning to her apartment with Philip after the party, she pretends to take a call from David. Claiming to have a headache, she then sends Philip away without a kiss. The next day, she fills her apartment with red roses, making it appear they are from David. However, her scheming is jeopardized when David is hospitalized for an emergency appendectomy and must cancel. Quickly revising her plans, Anna presses a reluctant Carl to wait in her bedroom in a dressing gown and instructs him to open the bedroom door, stand in the doorway briefly, then close the door after the first stroke of twelve. At precisely midnight, Philip arrives, says his wife has agreed to divorce him and asks Anna to marry him. As instructed, Carl opens and closes the door, but before Anna can explain, Philip leaves, believing that he saw David in her bedroom. However, he comes back, and upon recognizing Carl, accuses Anna of belittling their “fine and spiritual” love with a “cheap and shoddy trick.” Although she accuses Philip of lying to her, he maintains that he has behaved honorably and “stuck by the rules.” She argues that he proposed only because she made him jealous and he insists that he would have proposed--eventually. Sadly, she says they have lost their chance for happiness together and are not fated for marriage. When he disagrees, she suggests they continue as if the last two days never happened, but Philip is shocked that she would continue as before, unmarried. This makes her cry, but he consoles her by saying “You’ll like being married.”