Forensic scientist Abby Sciuto (Pauley Perrette) jets from D.C. to Los Angeles for an episode of "NCIS: Los Angeles" when a murder in the City of Angels resembles the pattern of a serial killer she has been tracking.

"Abby might get herself in a little bit of trouble," Pauley tells ET's Kevin Frazier, who visits her on set during her crossover episode. "It could be a little bit of Abby in peril. We have to think good thoughts for Abby, but she is pretty good. She has [gotten] herself out of bad situations before. She has been held at knife point, stalked, and she has been held at gunpoint at a nunnery, and she is still kicking, so now she is going to find a way to save herself in Los Angeles."

"She's coming in as a forensics expert," Chris O'Donnell, who plays Senior Field Agent Callen, tells Kevin. "We're on a case and Vance (Rocky Carroll) has told us, 'I'm sending in the best forensics expert we have.' She comes in and she has a theory about this phantom serial killer who's been murdering people around town."

This is Abby's first visit to "NCIS: Los Angeles." When the pilot for the spin-off aired as an episode of "NCIS," her character stayed in D.C. But she has already made a great impression on the cast.

"She's just a great shot in the arm," Chris tells Kevin. "She's got such great energy, personality and she's just a great spirit. From the get-go she's been really supportive of the show."

So will she be back for future cases?

"I don't know when I am coming back to 'L.A.,' or if," Pauley says. "I go where they tell me to go. And I am actually on both coasts this week. I am back and forth in real life. I am at two different studios shooting at the same time, often on the same day."

"NCIS: Los Angeles" airs tonight at 9 p.m. on CBS, after "NCIS," which airs at 8.