So really it's a continuation of the whole redemption arc from the last few episodes, where senior year is making everybody freak out about whether or not it's possible to be human, much less an adult, and after that it's just a domino effect, because Serena's having the same crisis here, not only for herself -- which she knows for a fact is now awesome, which is why she has to fix it herself -- but with Gabriel too, whom she spends the first half of the episode sort of stupidly hoping he can screw her over and simultaneously still be in love with her, because what really fucked Serena up is CeCe-via-Lily teaching her that you have the be the bestest friend to everybody in the world, because when you're tall and blonde and gorgeous and rich, people just want to fuck you up.
Which is true, and true not just for tall blonde rich girls, but doesn't mean you shouldn't watch your own ass and vote for yourself, which Serena's still not quite figured out. She still has the terrible twinned girl afflictions of Being Honest Is Being Mean, connected up to the Wanting Things Is Being Selfish thing, which are lies mothers whisper to their daughters as they lay them down to sleep, and which mean getting cornered a lot by brigands and Humphries and the like. Aaron Rose was all about not wanting to be the bad guy, for example.
If you were making a list of the psychoses of American teenage girls in 2009 you'd automatically go to Blair's million because she's shiny, but the really scary ones are the ones Serena quietly still has, because they're the normal ones nobody teaches us to get rid of: don't be unliked, don't be proud, don't be smart, don't talk too loud, don't ever say no. (Nate has the same set, for the same reasons, which is why they act out in the same ways: like wounded animals. Who occasionally bone in public.) And the most fucked up thing about all of this is that Lily's children have the most well-constructed morality of anybody on this entire show, because they actually sat themselves down in the middle of going crazy and thought their way there, which is how it happens.
So Serena calls Blair, because now she's got to come up with a whole new plan to fix the whole new mess, because Lily went from being somebody she could trust back to somebody who holds the keys to Serena's ability to think objectively about herself. This of course means that Blair can launch herself like a bullet from a gun back out of the Murray Hill apartment, where she's been standing for so short a time considering Nate's offer that she hasn't even taken off her coat. Nate offers protests as she goes screaming to Serena's aid, and they stare dewily at each other, and she begs him to leave it, and then he asserts himself in the only way you can without getting hit, which is to jump on for the ride.