AceShowbiz -

In the "Gossip Girl" theme of "old habits die hard", Blair and Serena will find themselves in the middle of a cat fight. On her first day at Columbia, Blair attempts to gain membership into an ultra-exclusive social club, Hamilton House, but she's in for a few big surprises.

Serena begins to suspect that Juliet (guest star Katie Cassidy) is intentionally sabotaging Serena's life and that it may have something to do with Nate. Rufus is worried about Dan and decides to take matters into his own hands.

Cassidy has signed up for big chunk of season 4. "This is definitely a big story for us," said executive producer Stephanie Savage. "Katie's with us for a good long time - at least for the first half of the season."

Speaking of Juliet's deal with Nate, Savage revealed, "The relationship definitely starts off as something with an agenda behind it, but by the end, it's real for both of them. Isn't that always how it happens? You start a relationship trying to use somebody and you fall for them every time."

"The Undergraduates" airs September 27 on The CW.