<Tr> <Td_colspan="2"> List of Gossip Girl episodes </Td> </Tr> <P> "Gaslit" is the 75th episode of the CW television series, Gossip Girl, as well as the tenth episode of the show's fourth season . The episode was written by Robert Hull and Joshua Safran and directed by Tate Donovan . It aired on Monday, November 29, 2010 on the CW . </P> <P> Gaslit is the fourth Thanksgiving - themed episode of Gossip Girl and has received positive reviews from critics . The episode deals with the gaslighting of Serena van der Woodsen (Blake Lively) as she copes with her supposed return to drug and alcohol abuse, the continued reluctance of Blair Waldorf (Leighton Meester) to resume a troubled relationship with her former flame, Chuck Bass (Ed Westwick), and Jenny Humphrey's (Taylor Momsen) short - lived return to the Upper East Side as she and Vanessa Abrams (Jessica Szohr) face the consequences of working with Juliet Sharp (Katie Cassidy) in taking down Serena . </P> <P> The episode opens with Blair discussing her activities with Dorota during Thanksgiving . Blair remains firmly adamant against Serena for what she did to her and Chuck during his party from the previous episode and plans to leave New York to spend Thanksgiving in Paris with her father . Dorota slowly persuades her to stay as she mentions making Blair's father's pumpkin pie, thereby persuading Blair to send it to the van der Woodsens by herself, hoping to make Serena apologize for what she had done . At the van der Woodsen apartment, Lily (Kelly Rutherford) releases her frustrations against Serena seemingly dropping out of college along with Eric (Connor Paolo). Throughout the city, Dan and Vanessa discuss spending Thanksgiving together while Nate (Chace Crawford) incidentally receives his parents' divorce papers . As Blair and Dorota arrive at the van der Woodsen apartment, she hands the pumpkin pie to Lily to honor her yearly traditions . When Blair asks about Serena, Lily questions her whereabouts as well and everyone realizes that Serena has been missing since Chuck's party . Meanwhile, a drugged and distressed Serena wakes up in a motel filled with pills and liquor bottles and calls 911 for help . </P>

When does everyone find out serena was drugged