At this point, Julien is accompanying Madame Fervaques to the opera in her private box. They discuss things like music and novels together. Julien is pleased to find that Madame has a bit of an obscene vein in her.
The next day, Julien is almost caught in a lie. In one of his letters to Fervaque, he forgot to change the names of certain places that Prince Korasoff used in his letters. He manages to lie his way out of it, though. Incidentally, it's the first time Madame has ever acknowledged the fact that she's read his letters.
Mathilde responds to this apparent romance by paying more attention to Croisenois, the man who wants to marry her. This kills Julien, but he's not willing to back down until he's done everything Korasoff told him.