Serena and Blair chat on the phone while getting dressed and trying on awesome dresses. Blair tells her about the breakup, and Serena asks if Blair's cheating on her with Vanessa and that's where the information is coming from, but Blair admits she's been spending "a lot of time with Nate." Her dreamy, seductive voice puts Serena on high alert, and she laughs and asks if this trip down memory lane might not just be another Chuck thing. You can tell Blair's in a good mood and thinking this fantasy is going to come true, because if she were at all nervous or anguished about it she'd be like, "What, are you going to fuck him again?"
There's a neat little thing here where Blair keeps trying on different dresses, all wildly different styles, as they talk: flowery yellow dress for her future with Nate, sophisticated scary red dress for her relationship with Chuck, etc. "I thought Chuck was my future, Nate a part of my past. But now I'm thinking Chuck was just something I had to get out of my system in order to realize Nate was my destiny all along." Serena indulges the little crackhead like always, all, "Sounds great if that's what you're thinking this week," and sweetly admits she wasn't exactly "rooting for Chuck." Which is not true, exactly, because all she does is root for Blair, and Chuck is good for Blair in some bizarre way.
Specifically, Chuck forces Blair to be actually present for her life instead of trying to climb into every movie she can think of, which is something Serena totally gets because that's her whole thing: actually being yourself for more than just a moment at a time. And no matter how much she explains it to Blair, and no matter how many times Blair understands it for a sec before reverting to her perfect fantasies, it's only Chuck -- or more properly, the uncontrolled and abject sense of falling that he symbolizes for Blair -- that actually can get her there.
So you have both girls trying to be grownups tonight, because somebody they thought was their equal -- and a prop in their story -- pulled the rug out from under their self-conscious belief that they were mature and in charge of their lives. Which is really what leaving high school is about, and it's a screaming mimi of a trainwreck but really fun to watch. It's so weird to see Serena and Blair in the same emotional place at the same time, because suddenly nobody is driving the bus. Add to that Jenny's whole thing, and this newest Rufus/Lily wrinkle that's about to drive her crazy, and add to it Dan's minor complications and Nate's ongoing van der Bilt crisis, and the only stable people on the show are Chuck (as usual) and Vanessa (for the nonce). Which is just so scary! Even Eric is getting middle-childed hardcore in this episode, because he loves Lily, Jenny and Serena equally, and not even his supernaturally perfect ass can figure out a way to keep them all happy, so he ends up fucking things up worse by trying. When even Eric van der Woodsen can't fix the problem, you know you are in trouble deep.