Hello, Nathaniel. It's been a while. Not even Lola is immune to the charms of Nate Archibald in a St. Jude's jacket, as he accosts her while she's trying to do her job and explains that basically the entire world is just The Matrix and wherever she goes he can find her because he and his fellow one-percenters have the secret cheat codes to everything so she'll never be safe.
As if by magic, Ivy somehow turns up -- apparently she bumped into Dan downstairs while his hipster ass was pretending to be into St. Vincent -- and Serena jumps up her skirt in that loud, weird way she always does with her fake cousin. Ivy's cagey and weird, also as usual, but lies that she went to Florida after the car accident for reasons that... Still don't come up in any way, even though she keeps promising to explain everything.
Blair randomly starts undressing Dan in the middle of the party and sends him after Serena, and he begs her to stop, and she pretends that he's just nervous about being in love with Serena: "Just be your charming, wonderful self, like you've been with me all year."
Dan: "You think I'm... wonderful?"
Blair: "Well, you've certainly been handy during my ongoing psychotic break. Plus the vows, and you saved me from my own wedding... If you're that guy with Serena, how could she not love you?"
Dan comes very close to kissing her, and she couldn't want it more, but right then GG texts him to kiss her, which ruins everything. He takes off to get self-righteous via text, and Blair gets enough ahold of herself that she goes to drag Serena back into the mix. This is the part where the entire episode becomes wonderful. Not just because of the Dan and Blair part, but because so much of the first half was getting the pieces in order so that the downward slide that starts here can just zoom on ahead without any time wasted.