As Blair's 20th birthday approaches, Nate and Serena have Blair and Chuck sign a peace treaty preventing them from plotting to destroy one another.
Dan decides to take Chuck and Blair down as revenge, and also to bring Jenny back to celebrate Lily and Rufus' anniversary.
Serena is still trying to resist Professor Colin. Six whole weeks is just too long. Juliet learns about S' woes and gets S to agree to let Juliet "chaperone" her at Blair's party.
Her job? Distract Serena when Colin's around. Nate convinces Serena be her "chaperone" instead, which messes up Juliet's plan.
Nate has to work hard to calm S down as she sees Colin flirt with someone else.
Nate sees Colin and Juliet, then assumes they're dating and that Colin is Ben.
Turns out Colin is Juliet's cousin and isn't in on this ... well, yet.
Dan, having stolen the peace treaty and apparently read some juicy details, convinces a record exec named Rita that Blair's party is actually a roast.
Rita arrives with a video of Blair drunkenly serenading Chuck after a Robyn concert. This video humiliates Blair so much that she rushes away.
Believing this was Chuck's doing, Blair confronts him until Dan reveals he was behind it. Rufus is disappointed he's become "one of them."
Juliet sneaks away to speak on the phone with Ben, who tells her that even if Colin, their cousin, is the professor in question, they have to use Serena's relationship with their cousin to ruin her. Get her caught with her professor and get her expelled. End of story.
Serena and Colin decide not to sleep together, but they do kiss in his apartment. Juliet managed to install a camera there to capture it.
Chuck emerges after Blair's party ends to tell her the deal is over. Then they tear each other's clothes off. Give it up for enemies with benefits!