If you're thinking, "Boy, these characters spend a lot of time talking and walking and not much time running away for some illicit underage sex," then this chapter is for you: Lizzy gets two letters from Jane saying that Lydia has run off with Wickham.
The first letter says that they've gone to Scotland, where it was a lot easier to get married without, you know, permission.
And that's bad enough, but the second letter is worse: they haven't gone to Scotland but to London, i.e. Wickham isn't planning to marry her at all.
The idea is that he's convinced her to have sex with him by promising that they'll be married, and eventually he'll abandon her. She'll never be able to marry a respectable man, and her sisters might not, either.
Lizzy's just freaking out when Mr. Darcy comes in. She knows he'll understand, so she blurts out the whole story in tears.
This is majorly bad news for the Bennet family, who are going to be shamed for having a daughter run off like that.
It's also bad news for Lizzy, since Darcy is never going to be okay with marrying into a family like hers.
Elizabeth and the Gardiners head home immediately.