Bartley Alexander is a construction engineer and world-renowned builder of bridges undergoing a mid-life crisis. Although married to Winifred, Bartley resumes his acquaintance with a former lover, Hilda Burgoyne, in London. The affair gnaws at Bartley's sense of propriety and honor.Professor Wilson arrives at the Alexanders' house in Boston, Bartley Alexander having persuaded him to attend a Congress of Psychologists in the city. He is greeted by Winifred Alexander. When her husband comes home the men talk; Winifred plays the piano for them. The next day, she tells Wilson how she met her husband through her aunt.
On Christmas Eve, the Alexanders prepare for Christmas dinner. Bartley tells Wilson he is having trouble with a bridge in Canada. Later he gives his wife pearl earrings. On New Year's Day, Alexander makes ready to leave for London. On the ship, he endures sharp gales and goes into a bar, where he gambles at bridge, the card game. In London, Bartley visits Hilda and tells her he cannot go on having two relationships; she must forget about him and leave him alone. She is distressed. The day before he is due to return to America however, he takes her out to dinner.
Later, Hugh MacConnell walks Hilda back to her house on a foggy day. She says she isn't attracted to him; they are just close friends. In her house, she receives a letter from Bartley, saying he is going mad away from her. This prompts her to visit him in America to tell him she will marry another man; Bartley doesn't like the idea. They spend one last evening together.
Soon afterward, Philip Horton calls Bartley to Canada to inspect the bridge. Bartley discovers that one of the lower chords is failing, compromising the structural integrity of the entire bridge. Horton, concerned not to halt construction, had attempted to contact Bartley earlier â the very day Bartley was with Hilda. As Bartley is on the bridge stopping the work crews, the bridge collapses, killing many of the workers. Bartley's body is recovered the next day and taken to Horton's house. Winifred comes back to make arrangements for her husband's corpse. Finally, Wilson visits Hilda. The latter expresses her envy of Winifred. Wilson reminds her that Winifred is now bereft and will be haunted by Bartley's death. Hilda concludes that she will be too.

Question: Who is Bartley's mistress?
Answer: Hilda
Alternative answer: Hilda Burgoyne.