Blair and Dan run into each other at Registration, checking in about the mysteriously vanished Serena, who has barely been heard from since Thursday, you remember, and Dan's like, "No idea where she is, but I bet Chuck does." B nods, making the obligatory Chuck-is-scum face, and offers to save her a seat. Dan dials her phone again, leaving a very cute and very Dan message ("Hey, Serena, it's me--a very worried me, concerned about a very missing you..."), but breaks off as a lovely redhead checks in at the desk: "Hi. I'm Serena Van Der Woodsen?"
After the test, B continues to look freakishly amazing, and Dan admits to her that, choker or not, this wasn't his finest hour: "I finished, but... I kinda had a lot on my mind." She nods, as comforting as she ever is with him: "I'll keep calling." He wonders if it's actually necessary to go to the Bass suite, decides to head over, but stops about two steps down: Vanessa and Nate, laughing, oblivious, getting into Nate's towncar. So now S is off doing God knows what with Chuck, and Vanessa's creeping around with Nate Archibald, and I do love Dan, but I mean: that's all of it. He defines himself by these relationships, and now the two most important women in his life -- three most, with Jenny also schizing out -- are prancing merrily away into other stories, without even mentioning it to him. "Hey, S. It's B. Now I'm really worried. I'm gonna go home and change, and then..." She sees what Dan's staring at, and joins him downstage, remembering the phone in her hand after a moment: "Call me. Bye." He looks at her, realizing what this means to her too, and the mysterious couple drives away. And she doesn't look at Dan exactly, but I like to think she takes a little of the warmth he's directing at her, as she goes. She accepted Dan taking parts of Serena away from her, she's Blaired that out and made it work, but every time she sees Nate it's like being slapped, and this new girl? After all that time trying to protect Serena from Vanessa, and now the call is coming from inside the house: she forgot to protect herself.
Dan knocks on the door at 1812, and S looks at him through the peephole sadly for a bit before finally opening it. And when she does, there's no interrogation and no resentment, nothing but relief; he bends into her. "Hey!" He touches her arms; she is still and afraid he'll smell it on her. Afraid he can somehow see Bad Serena, lurking around, waiting to come out, waiting to take him down. "What's going on?" She steps away, shuts it down, buttons up like Lily taught her: "Um... Nothing. I'm fine." He shakes his head, getting scared. "No, you're not, and you need to stop saying that. Hey. Come here. Where were you?" Chuck steps out of the shadows, ready to play, unaware that anybody as vulnerable as Dan even exists: "She was with me." Serena shakes her head. "What he means is, we were both at home. Um, my stomach started acting up again." Dan gets a little irritated and begs her not to lie: "Look, if you didn't study enough and you're freaked out, I can..." Because at this point, he has more parts of the puzzle than she does, because she doesn't know about the redhead, so she's playing this all wrong and making it look even worse than it is, she gets defiant: "I told you. I was sick."