S4, E3: It's back to the Upper East Side! On her first day at Columbia, Blair attempts to gain membership into an ultra-exclusive social club, Hamilton House, but she's in for a few big surprises. Serena begins to suspect that her life is being intentionally sabotaged by Juliet (recurring guest star KATIE CASSIDY), and that it may have something to do with Nate. Meanwhile, Rufus is worried about Dan and decides to take matters into his own hands.