The younger girls are depressed because the military regiment (i.e., the attractive men in uniform) will be leaving soon.
But good news! The wife of the regiment's colonel invites Lydia to go to Brighton with them.
Brain Snack: Brighton is a beach town and really fashionable in the Regency, so it's kind of like heading to, oh, Ft. Lauderdale for spring break or something: parties, drinking, boys trying to get you to take your shirt off, that sort of thing.
Right, so you can tell that spring break + Lydia = really bad idea.
Lizzy points this out to her dad, but he's unswayed. He says it's better to let Lydia get her kicks out in Brighton rather than in Meryton, where she'll embarrass the family.
Uh, Lydia already embarrasses the family, she says.
Mr. Bennet can tell she's upset, but he says that Jane and Elizabeth's reputations cannot possibly be hurt by their three very silly sisters.
It's settled: Lydia's off, and Elizabeth can finally say good riddance to Wickham. He's really lost his charm for her.
She does get one dig in: she tells Wickham that she spent three weeks with Mr. Darcy and Colonel Fitzwilliam. She lets Wickham know that she liked the colonel immensely, and adds that Mr. Darcy's personality and manners improve as she gets to know him better.
This is a really subtle way of telling him that she knows he's a lying liar.