The summary of Roman Fever is tricky, I find, because the women's names tend to swim together in my mind and their identities become blurred and confused. Having said this, the theme of Roman Fever is actually that their identities did become confused. So, I'll approach giving a summary in a slightly unorthodox manner that you, once you absorb it all, can render in a more orthodox form if you wish. First, Mrs. Spade married a corporate lawyer; henceforth for clarity we'll call her Mrs. Corporate. Mrs. Ansely is said to represent old museum aspects of New York society; henceforth we'll call her Mrs. Museum.
While they are talking the afternoon away, each harboring a subtle inclination to linger although they are dissimilar and not altogether comfortable with each other, Mrs. Corporate brings up an afternoon when they were young and visiting Rome together along with Mrs. Corporate's then fiance, whom we will call Mr. Corporate. Mrs. Corporate reminds Mrs. Museum that on that day, Mrs. Museum developed a serous illness that kept her in bed for weeks. Mrs. Museum is hesitant to acknowledge recollection until Mrs. Corporate bursts out with the statement that she knows why Mrs. Museum got ill.
Mrs. Corporate says to Mrs. Museum that her illness was because she received a rendezvous letter from Mr. Corporate (someone else's fiance...!) but that he had not written it--Mrs. Corporate had penned it herself as a wicked lure to ruins of the Forum to cause Mrs. Museum's illness in order to keep her out of the way of the romance between the then soon-to-be Mr. and Mrs. Corporate. Mrs. Museum says, hmmm, you wrote it...well...I answered it, and Mr. Corporate was there at the rendezvous to greet me...so there.
Mrs. corporate says, well, it was long ago and now you have only the memory of one night of chit chat. Mrs. Museum says, oh, contrare, mon chere...I have my daughter...Barbara. The anvil falls and crashes on Mrs. Corporate's head. Mr. Corporate was Mrs. Museum's willing lover and the father of Mrs. Museums daughter. This is a situation of painful, tragic situational irony: The duper turns out to be the self-inflicted victim of the dupe.