Chuck and Blair are faced with the most difficult decision they have ever faced as a couple. Serena plans a surprise birthday party for Nate, but Jenny does her best to sabotage Serenas’s efforts. Dan and Vanessa’s new relationship hits a bump in the road.

Chuck offers to throw a wedding for a very pregnant Dorota who insists that she and Vanya have a traditional wedding before her parents arrive from Poland and find their daughter pregnant and without a husband.. Dorota asks Blair and Chuck to be part of the ceremony and walk her down the aisle. Serena tells Nate she is having breakfast with the Humphreys when in reality she’s secretly meeting up with Carter Baizen. Rufus discovers that Lily has been lying to him about her whereabouts.

When Lily’s ex-husband, Dr. William van der Woodsen, shows up unexpectedly in Manhattan, Serena finally gets the reunion she has so desperately wanted with her father, but the circumstances turn out to be far from ideal. When Serena and Nate continue to have relationship issues, a scheming Jenny is more than happy to be a shoulder for Nate to lean on... Meanwhile, Dan finds out that he didn’t get accepted into the Tisch writing program and Vanessa admits she may have been somewhat responsible.

Will's return to the Upper East Side continues to stir up problems for Serena, Lily and his long-standing rival, Rufus. Serena and Jenny's relationship is strained as a result of Jenny's deception. Blair spends time on the Columbia campus and realizes where she truly belongs.... Dan learns that Vanessa has been keeping yet another secret from him.

Blair must decide whether to meet Chuck at the top of the Empire State Building or risk losing their future together. A Gossip Girl gossip bomb involving Dan explodes Serena and Nate's relationship. Dorota and Vanya welcome their baby. Jenny hits rock bottom. Georgina returns to the Upper East Side...and she has an agenda.

Chuck hosts a "Saints and Sinners" masquerade party at the Empire Hotel. At the party, Dan and Nate don't know what to make of Serena's strange behavior and mixed messages. Blair sets her sights on becoming the new face of Anne Archibald's foundation for girls, but Anne is concerned that Blair's friendship with Chuck Bass may be problematic.

Serena enlists Charlie to go undercover Gossip Girl-style to determine why Dan and Blair are spending so much time together, and what role an unexpected visitor is playing in their lives. With Nate’s help, Raina decides to go on a search for her long-lost mother. Chuck’s hopes about his legacy are dashed with the reveal of devastating information from the past.