The themes of rivalry, love, passion and jealousy are important in this story. It does have a very tight plot- a lot is revealed in a short amount of time by very concise dialogue. If you'll notice, the conversation the two women have is often not even in complete sentences. Look how Grace Ansley respondes to Alida Slade. She often only blurts out a word or two. We see what the two are thinking, but their thoughts are brief, concise but full of meaning. The two women each have different and wrong assessments of each other in the beginning. This builds as the story progresses. Alida Slade reveals a mounting jealousy for Grace Ansley, even though Alida has really had a more exciting life, been married to a more handsome, successful man, etc. This conflict builds until ultimately, Alida blurts out why she is jealous. We find out that she suspected Grace was in love with her husband Delphin years ago, in Rome. She is correct. She tried to trick Grace into going to the Colosseum to meet Delphin by writing her a phony letter from him. Grace answered it though, and because Delphin loved Grace as well, he DID go there to meet her, they made love, and Grace had Delphin's daughter. Alida thought she had made her friend ill by luring her to the Colosseum to wait in the cold, thus catching a chill, and getting sick. But Grace tells her that she did not wait, Delphin was there, and "she had Barbara." That is the climax and also the ending.
The jealousy that Alida had all these years wound up turning on her. The rivalry that she had with her friend that caused the jealousy turned out to be well-placed, but not for the reasons Alida thought. Grace has had a night of passion, the memory of which has lasted all these years. Alida has had a night of vengeance which turned on her, especially in the end, when she realizes that Barbara, Grace's daughter that she so admires, is really the daughter of Delphin, her husband.