Meet Jess' Worst Nightmare on New Girl—Her Dad's Very Young and "Loose" New Girlfriend!

Exclusive! It could have a little something to do with the fact that she's dating Jess's dad and is a sex addict. Maybe.

It's always difficult when kids watch their divorced parents date other people. But Jess (Zooey Deschanel) is about to live through her worst nightmare on New Girl when her dad (Rob Reiner) brings his new girlfriend Ashley over to the loft.

Because his new arm candy is none other than It's Always Sunny in Philadelphia's Kaitlin Olson!

While we would be jumping up and down at the chance to welcome Olson into our family, Jess is less than thrilled for a couple reasons:

1. Ashley went to high school with Jess and is the same age as Jess.

2. Ashley was...ahem, loose, in high school. And still is.

And even though Jess is not happy with her dad's choice in love interests, she's going to try and put her best foot forward when it comes to welcoming Ashley to the loft. That is, until the word "wedding" starts getting thrown around, and then all bets are off!

We've got an exclusive first look at Olson's amazing guest-starring role on New Girl, and trust us when we say that an angry Jess is a hilarious Jess.

Watch the exclusive clip above now to see just how mad Jess can get!

New Girl airs Tuesdays at 9 p.m. on Fox.