Anne is at a loss as to how Blair, in her gorgeous underwear, can possibly explain this situation as a sign of dedication to G.I.R.L.S. Inc. -- which of course B doesn't know about -- and anyway, Anne hates her now that she was kissing Charles in her underwear. "Shouldn't empowered women get to have relationships, too? Being with Chuck doesn't make me less Blair." Anne decides to play Joan Holloway for a second and explain some incredibly basic shit: "I'm sorry, but it does. The man you're with may not be a reflection of you, but you will always be a reflection of him. Maybe not [fair], but the sooner you accept it the easier your decisions will be."
Blair is still having Jezebel-type "reality" v. "my preferences" trouble, so she's a little distracted by Chuck heading over to get reamed by KC, and then she remembers that Serena did a dick move for no reason, and goes looking for her. But I mean, if that were true then we would all be dating Nate, because nobody else reflects well on anybody. Girls, boys, indifferent: The only person on this show you should date is Nate. Until Eric graduates, and then oh man.
Serena is trying to explain to her boyfriends that she was locked out of the party, while somebody dressed exactly like her sidles up and puts roofies in her mask without anybody noticing. And so, they ask, how come "someone dressed exactly like you, wearing your perfume" and of vastly different body types and sizes, happened to be there to kiss both of them? She can't explain it, so fuck them. And then actually the boys make a good point which is that she already pulled this group-text thing with them from Paris, and then again with Colin, and then again after Colin, and has stood them both up before. Which is also a good point, and also stated in a way where she can't point to it as part of the conspiracy. She leads up to her grace note, which is that she has finally picked, and they're both like, "WE DON'T CARE ANYMORE" and take off. Which is exactly what happened when she got home from Paris, just without the baby or Rufus here. Plus date rape drugs.
Blair shows up to punch her face and Serena swears she didn't do all that kissing, and Blair's like, "I don't know, maybe you did! You sabotaged me and Chuck!" Serena doesn't get that she's talking about the curtain-pulling and thinks she is somehow to blame for B showing up in her underwear and getting yelled at by Anne, and then B brings up the other lie, about how she was suddenly trying to get involved with Going Into Rehab is a Last-Ditch Save, Inc.