Credit: Courtesy of the CW

It's her party, so she can have drama if she wants to! A surprise guest catches Blair (Leighton Meester), shown here in a Giambattista Valli suit, Chris Benz top and Paige Gamble headband, off-guard—and not in a good way. Having had enough of Chuck (Ed Westwick) and Blair’s games, Serena (Blake Lively) and Nate (Chace Crawford) decide to plan an intervention. Musical guest Robyn also performs. Tune in to the CW at 9 p.m. EST Monday night to watch the dramatic episode, and click through the photo gallery now for head-to-toe credits of what everyone will be wearing!