NCIS: New Orleans, NCIS Crossover: Maren Morris, Gregorio's Secrets and the Return of Eva

NCIS: New Orleans and NCISteam up for another blockbuster crossover episode Tuesday, with the action taking place amid the hustle and bustle of Mardi Gras and a special visit from Grammy nominee Maren Morris, who will perform "My Church."

But there's much more going on than music, of course; in the first half of "Pandora's Box," Gibbs' (Mark Harmon) NCIS team finds that a Homeland Security "theoretical terror playbook," which plays out how personnel would respond in attack scenarios, has been swiped. When they learn it has landed in New Orleans, Agents McGee (Sean Murray) and Torres (Wilmer Valderrama) are sent to the Big Easy to investigate.

Naturally, that puts them in the house that Agent Pride (Scott Bakula) built, and they'll be partnered up with Sebastian Lund (Rob Kerkovich) and Agent Tammy Gregorio (Vanessa Ferlito), respectively. While Sebastian and McGee strike up a rapport right away -- due in large part to Sebastian's admiration for McGee's tech skills, and his very similar rise from rookie to butt-kicking agent -- there's initial friction between Gregorio and Torres.

Check out Maren Morris performing "My Church" on NCIS: New Orleans

"One of the fun elements of getting to bring two characters down to New Orleans is the ability to play with team dynamics, their similarities and differences," Chris Silber, a co-executive producer on the series, told TVGuide.com. "Torres is a great example. He's brand new and Tammy is pretty new to the New Orleans team. They're both strong-minded, action oriented characters. There's tension between them for sure but they also find common ground," he said. They commiserate about their adjustments fitting in, and learning to trust. Trust is of special significance for Gregorio, who'll reveal more about the situation with her ex-husband who caused her to flee New Orleans in the first place.

You'll also see the reappearance of Eva Azarova (Cassidy Freeman) who you'll remember from the previous collaborations between the two shows is affiliated with shady Russians and had an uneasy alliance with Pride. Just as they were back then, her motivations weren't entirely clear. "When we last saw her she was on the way back to Washington. She's very much involved in the plot having to do with the playbook and these series of scenarios of how to attack the United States. She plays a big part."

NCIS: New Orleans airs Tuesdays at 10/9c on CBS.

(Full disclosure: TVGuide.com is owned by CBS.)