"Are you kidding me?" Montag says after spotting her beau with a bartender he was caught flirting with last episode. "I had to find out that you were here sneaking around with Stacie. Are you guys dating? Are you having more shots?"
Charlie, a friend of Pratt's, steps in.
"Heidi, why don't you chill out?" he says. "Your man is just having a night out. He hasn't done anything."
Pratt's younger sister Stephanie came to Montag's defense, but Pratt -- who says that they all just met "five minutes ago" -- tries to get them to sit down.
"Can I join this double date because that would be so much fun?" Montag says. "How skeezy!"
After the altercation, she demands that she and Pratt go to couple's therapy.
Also on the new episode -- which airs Monday at 10 p.m. EST -- of The Hills, Lauren Conrad puts her reputation on the line when she gets Stephanie an internship at People's Revolution.
Tell Us: Whose side are you on -- Heidi's or Spencer's?