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Blair is caught off-guard -- and not in a good way -- by a surprise guest at her birthday party. Having had enough of Chuck and Blair's games, Serena and Nate decide to plan an intervention. Musical guest Robyn performs in the episode. Penn Badgley, Taylor Momsen, Jessica Szohr, Kelly Rutherford and Matthew Settle also star. Joe Lazarov directed the episode written by Jessica Queller.