THE SUMMER IS OVER AND SERENA HAS GONE MISSING / BARRY WATSON (“7th HEAVEN”) AND ANDREA GABRIEL (“LOST”) GUEST STAR – After a brief encounter abroad, Chuck and Blair make a promise to each other about their relationship, which leaves them once again apart. Dan, with some help from an unlikely ally, Georgina (guest star Michelle Trachtenberg), is writing a scathing tell-all book that can only mean trouble. As the sole head of The Spectator, Nate is making his own decisions and is ready to go up against Gossip Girl. As word spreads that Serena is missing, the gang bands together to find her and are shocked by what they find. Kelly Rutherford, Matthew Settle and Kaylee DeFer also star. Robert John Burke and Sofia Black-D’Elia (“Skins”) guest star. Mark Piznarski directed the episode written by Stephanie Savage and Josh Schwartz.