Pride and Prejudice Volume 2, Chapter 18
Lydia is invited to accompany a colonel and his young wife to Brighton with the soldiers. Mrs. Bennet is as ecstatic as Lydia, although Kitty is pouting. Elizabeth sees the danger in letting Lydia go with the soldiers, and so she speaks to her father about it, pointing out the potential for great embarrassment and shame for the family if Lydia misbehaves. Mr. Bennet assures Elizabeth that anyone who knows her or Jane will not judge them based on the actions of one of their silly sisters, and, despite Elizabeth's pleas, he lets Lydia go to Brighton.
At a farewell party the night before Lydia and the soldiers leave, Wickham talks with Elizabeth and she mentions seeing Darcy and Colonel Fitzwilliam on her visit to Rosings. She is trying to scare him off because he seems to be trying to renew her fondness for him since he's dumped the ugly heiress. When Elizabeth tells Wickham that knowing Darcy better makes him more likeable each time they meet because she understands his personality better, Wickham takes the hint and leaves her alone. He's not sure just how much of the truth about his relation to Darcy that Elizabeth knows.