Brody and Kristin's 'Friends with Benefits' Plan Goes Awry on 'The Hills' (VIDEO)

The "friends with benefits" agreement between exes Kristin Cavallari and Brody Jenner isn't going very well -- for Kristin, that is. It works for Brody. (Who's surprised?)

On 'The Hills' (Tue., 10PM ET on MTV), Brody introduced new girl McKaela to the gang, and Kristin was far from amused.

"Just out of respect for each other, I would never in a million years bring around some guy and be all over him the way that you were all over McKaela in front of [me]," said Kristin. "Friends don't do that sort of thing to each other."

"Exactly! You just said it -- friends," answered Brody. "Girlfriend and boyfriend don't do that to each other."

They continued to argue and it didn't end on a good note. As Kristin headed for the door, Brody said, "We're friends, Kristin, remember that." "Shut up," she responded.