He storms past, because of course Serena should magically have known that two months after dumping her for literally no reason he would appear at this random tacky party and she could not under any circumstances be spending time with a friend she's known since grade school, much less kissing anybody. And the reason for that is that he took the jitney, so he was on that bus for hours just thinking about how perfect it was going to be when he finally showed up and forgave her for the imaginary thing he needed to forgive her for, and she'd throw herself down at his feet and apologize for being rich and promise to make it up to him, no matter what he wanted, and she'd be grateful for it. Instead, she's having fun, because Serena does not give a fuck. She cares deeply about Dan and about mooning around, but now that she's worked out the whole murder thing for herself, he's got nothing on her.
Of course she immediately goes into fixer mode, for the third time this week, running after him all, "Wait, wait!" And he tells her no thanks, while doing a curious thing, which is standing very, very still, as if he thinks he's creating the illusion of walking away simply by saying no thanks. Serena points out that he appeared out of nowhere after two months, and surprised her, and he retaliates with the very nasty "I have to say you didn't. Nate, huh? I guess even bad history repeats itself." So, the girl you came to the Hamptons to find, you managed to call her a whore within the first three sentences. I have to say you're not really shocking me either, Dan. Slap him! Slap him silly! Slap him so hard he can't even order a Vitamin Water so he dies of dehydration.
Serena swears there's an explanation, she was helping Nate make somebody jealous, but she can't say who and she can't say why, which is not the point -- because the point is that Dan is being twice his usual cocksucker self from a position a tenth as valid -- and Dan says that no, still Serena is a whore because guess what: "It can't just be I kissed Nate. It has to be I kissed Nate because someone I can't name needed to see it for a reason I can't explain. It's the same drama, different city." Agreed, except all of that is also Dan's fault for being so manipulative and controlling in the first place that she feels like she has to hide and cover up and lie about the most basic shit in the fear that he'll throw a big fucking hissy fit about it ... which he's going to do anyway. He exits pompously off the "same drama, different city" line, and -- Thank God! -- runs smack into the two random girls from the reading. Long story short, they are stock footage in human form about how Dan's a dumb whore, then pour VITAMIN WATER on him. "Let me guess," Serena says -- in a not entirely arch manner -- "You can explain?"