In "Roman Fever," Edith Wharton expertly sets up the twist ending by spending a lot of time drawing such a sharp contrast in personality between Grace and Alida. Grace is presented to us as a mousy, unobtrusive character, someone we'd never suspect in a million years of engaging in an illicit love affair. By the same token, Alida is given to us as a dominant personality, bossy and imperious, someone used to getting her own way in life.

Once again, Wharton is preparing us to have our expectations well and truly confounded, for we cannot imagine someone like Alida having the tables turned on her so comprehensively, let alone by someone as mild and inoffensive as Grace. The vivid characterization of the two ladies makes the twist ending not just surprising, but also rather satisfying. After all, who doesn't like to see someone as haughty and smugly superior as Alida get taken down a peg or two?

Grace Ansley is an upper class wealthy widow who has led a comfortable, quiet life with her husband.  According to Mrs. Slade, Mrs. Ansley's life was boring compared to hers.    

Alida Slade is a powerhouse of a woman, the wife of a famous lawyer.  She was her husband's hostess and travelling companion throughout his exciting career.  Now a widow, she misses the limelight that being Mrs. Delphin Slade used to provide.  She is a resentful type, unhappy with her present circumstances.  Mrs. Slade is in the midst of an identity crisis, so her long held jealousy of Mrs. Ansley easily rises to dominate her feelings on the vacation.

Mrs. Slade feeling the need to be superior and needing a little excitement in her otherwise boring existence, brings up the past when both women were young and in Rome on holiday.  Mrs. Slade's intention is to demean her; she wants to crush this woman. 

Alida Slade wants to punish Grace Ansley for ever having been in love with Delphin Slade.  So she tells her that she was the one who wrote the letter of invitation to her, not Delphin.  Expecting a shocked looked, Alida is handed a time bomb.

Grace informs Alida that she returned a note to Delphin and did in fact meet him that night so long ago.  She may have had a brief fling with him, but she also had his daughter, Barbara. 

Grace Ansley gets the upper hand as soon as she reveals that she replied to the letter and met with Delphin for a romantic rendezvous.