In a fashionable farewell to remember, our favorite Upper East Siders join forces for one last soiree, and the shocking identity of Gossip Girl is finally revealed.

The Constance Billard and St. Jude seniors have the production of "The Age of Innocence". Serena has a crush on the play's director, Julian, and enlists Vanessa's knowledge of theater to help her get the guy's attention. Blair receives bad news about her future that leads her on a witch hunt for the person responsible. Dan and Rachel agree to stay away from each other in the aftermath of being discovered as more than teacher and student, but find it impossible to keep that promise. Chuck find help on an unlikely friend, Carter Baizen.

The much-anticipated 100th episode will open with a musical dream sequence featuring Serena, Blair and all the men in their lives. Also, while Chuck wrestles with the reality that Blair is about to walk down the aisle with Prince Louis, the return of Georgina Sparks might make the royal affair a royal disaster.

In a flashback to the 80s, 17-year-old Lily Rhodes gets kicked out of boarding school and runs away to Los Angeles, reconnecting with her parents, Rick and Cece, and sister, Carol. Back in present day, Blair and Nate attend prom together, but the evening doesn't turn out exactly as planned and Chuck and Serena have a little something to do with that. Serena and Lily don't see eye-to-eye over Serena's recent run-in with the law.

Living out a real-life fairytale in Paris, Blair is pursued by a handsome young man, Louis, who she suspects is a member of Monaco's royal family. A very concerned Lily asks Serena to help her find Chuck, who hasn't been heard from or seen in months. Meanwhile, Chuck Bass is as we've never seen him before, adjusting to a fresh start in Paris with a mysterious new woman, Eva, at his side. Juliet offers to help Nate win Serena back, suggesting that reigniting Dan and Vanessa's romance may just be the trick to remove Dan from the Dan-Serena-Nate love triangle.

After Georgina reveals the true reason Serena left Manhattan, Serena goes back into her old habits. Her friends Blair, Nate, and Chuck, in order to help her, have to put their differences aside. Meanwhile, Rufus is excited when his band is invited to perform at a Rolling Stone-sponsored concert, but Lily is the last person on earth he expects to see at the performance, especially since her wedding rehearsal dinner is the same night.

Blair and Dan join forces to find Juliet. Serena expounds her boarding school experiences to her therapist and discovers the truth behind Juliet's vendetta against her. Meanwhile Chuck suspects Lily has not told him the full truth.

Serena and Dan finally accept that they are crazy about each other, despite that they come from polar opposite worlds. Chuck thinks of investing in a club trying to make his father Bart Bass proud of him. Nate confronts his father, about the drugs he found in his house but he denies having a problem. Jenny discovers a secret that her parents have been keeping from her. Finally, Blair is devastated by Nate's actions when Jenny reveals that Nate kissed her by accident thinking she was Serena.

Georgina informs Serena that she can reveal a deep dark secret from Serena's past, but no one is prepared for what Georgina does next. Blair and Jenny's power struggle continues when each of them start spreading scandalous rumors about each other on Gossip Girl. Dan sees Jenny's new beau, Asher, cheating on her, but Jenny won't listen to him. Rufus goes to Lily for advice on how to handle Jenny's new behavior. Serena tries to confront Georgina, but she leaves before Serena has a chance. After this, Serena says that she and Georgina are not friends.

The entire Upper East Side of Manhattan is rocked when Bart Bass dies suddenly in a limo accident. Serena and Eric take the news very calmly while Lily has a troubling time despite her mother, Cece's, awkward attempts to comfort her. Chuck begins to blame Lily for his father's death and threatens to reveal a secret that Bart found on her before his accident. Meanwhile, Blair's mother, Eleanor, prepares for her latest marriage to Aaron's father Cyrus.

The episode continues from the early events of the Constance Alumni event. Georgina Sparks turns up at the event looking for trouble. Serena and Vanessa reluctantly join forces to find Dan and Charlie, who does not take Dan's rejection of her well. Blair must make a choice between the men in her life when her life is in danger by Russell.

Lily throws Charlie a party to officially introduce her to New York society. Meanwhile, Charlie’s ex-boyfriend, Max, throws a wrench into Charlie’s future as an Upper East Side princess. Dan decides it may be time to tell Blair the truth about his feelings for her.

Lily and Rufus decide to move up their wedding to the next day, but will someone or something come between them before they become man and wife? Chuck discovers a secret that Carter has been keeping from Serena. Dan and Vanessa are pulled into one of Georgina’s schemes. Blair is suspect of Bree and shares her concerns with Nate.

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.

Chuck, Serena, Blair, Dan, Jenny and Eric find themselves faced with a crisis when a member of one of their families is hospitalized. Meanwhile, Nate attempts to help his parents, The Captain and Anne, put their complicated past behind them and reconcile.

Chuck, Nate, Blair, Serena and Lola team up to uncover what they think is an explosive secret between Diana and Jack. Meanwhile, Dan is offered a prestigious fellowship in Rome for the summer.

Serena's grandmother announces that Serena has to go to Debutante Ball. However, Dan is not what CeCe wants as an escort for Serena, and she makes her opinion heard loud and clear, causing a rift between Serena and Dan. Although they are officially broken up, Nate asks Blair to go to the ball with him. A jealous Nate begins to suspect that Blair is seeing someone else. Jenny finds herself having to make the decision of either attending her mother's art opening or following her dreams to the Debutante Ball, even though her parents don't want her to go.

Gossip Girl reports that Serena van der Woodsen was seen buying pregnancy tests. Of course, on the Upper East Side, the truth is often even juicier and more scandalous than the gossip. Nate finds out about Blair and Chuck's affair. And when everyone else finds out what happened Blair's life comes crashing down. Meanwhile, Dan struggles to tell Serena those three special words.

Having been recently dethroned as Queen Bee, Blair returns to school with the support of Serena to find she is still on the outs with her old friends. Dan visits his mother over spring break, and when he returns, Jenny is on the verge of becoming a full-fledged member of the A-List crowd. Jenny must go to dangerous and even illegal lengths to fit in. Serena struggles to adjust to living with Chuck, and receives disturbing packages from an unknown sender.

Blair takes matters into her own hands to help Serena with the manipulative and evil Georgina Sparks. Lily, while keeping Rufus in her mind, prepares for her wedding with Bart Bass that is designed to be the Upper East Side's social event of the year. Serena finally tells Dan the whole truth about her past and they try to work things out after Dan has cheated on her, but will it be too late? An unexpected guest shows up at Lily and Bart's wedding. Georgina is sent somewhere where she can do no harm. Blair plans to go somewhere with Chuck.

Vanessa uses the scandalous picture of Catherine and Marcus to blackmail Blair into being a nicer person, so Blair seeks revenge by enticing Chuck to seduce Vanessa. Lily and Bart throw an extravagant housewarming party and expect their children to make them proud in front of the attending press, but a rebellious Serena isn't ready to play by their rules. Meanwhile, Dan and Jenny discover a big secret that Nate has been keeping from his friends.

It's Thanksgiving time and Blair is offended when her Mom's new man hijacks her Thanksgiving traditions. Lily attempts to strengthen the frail bond between Rufus and Jenny. Eric discovers a secret about Bart. Vanessa and Chuck help Nate deal with his father.

Gossip Girl decides to liven things up at the Constance Billard and St. Jude’s graduation by sending out a shocking and damaging email blast in the midst of the commencement ceremony, stirring Serena to come up with a plan of attack. The Mean Girls challenge Jenny to throw her hat in the ring to succeed Blair as Queen Bee next year. Lily and Rufus struggle to repair their relationship after Lily’s betrayal. As for the cliffhangers, Blair and Chuck finally… and Serena is shocked by… and Georgina decides to… and Dan, Nate, and Vanessa unknowingly…. Jenny and Eric do something they might end up regretting.

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.

Blair and Serena spent their summer living it up in the belly of the beast: Paris, France. But where there's Paris there's bound to be boutique, Balenciaga, and of course, boy trouble. Unaware of Chuck's shooting in Prague, Blair struggles to forget him; things take a turn, however, when Chuck mysteriously shows up in Paris with a foreign stunning blonde at his arm. Meanwhile Serena contemplates which of her long-time beaus could be the one for her: Dan or Nate? Even on the Upper East Side drama erupts as Dan figures out the baby mama drama with Georgina, and Nate realizes something about himself when he meets fellow Columbia student, the intriguing Juliet.