Michelle Trachtenberg

A holy terror alert has been issued for New York’s Upper East Side, and here’s why: Georgina Sparks is headed back to town!

Gossip Girl exec producer Josh Safran confirms to TVLine exclusively that professional pot-stirrer Michelle Trachtenberg is returning to the show for a sizeable arc.

And far be it from her to slip in quietly in the night, Georgina’s reentry has been timed for maximum impact.

“Her first episode back is our 100th episode,” Safran reveals of the show’s milestone outing, which is set to air in early 2012. “Her arc is unlike anything that she has done before on the show. Of course, she still is Georgina and all that implies, it’s just taking that in a new direction.”

When last seen in the Season 4 finale, the now-married mother of one — bored to death by domestic life — returned to the UES looking for some trouble. What she found was an unlikely kindred spirit in interloper Charlie. (Ms. Sparks skedaddled shortly after discovering Charlie’s Big Secret, which is sure to make their reunion a tense one.)

But wait, there’s more: The sixth coming of Georgina promises to take on added dramatic effect when you learn what Gossip Girl‘s 100th episode revolves around — a burning question I will be answering this evening in the Nov. 1 edition of Ask Ausiello.

Hint: It’s a doozy.

Until then, feel free to welcome — or unwelcome — Georgina back in the comments!