NCIS: Los Angeles is back, but Sam and Callen are still in "Deep Trouble" during the season 6 premiere.

Inside Kensi & Talia's 'NCIS: LA' Fist Fight, Daniela Ruah Explains Premiere Battle

The two agents spent the summer hiatus trapped in terrorist's bomb-laden submarine bound for San Diego, and the clock will continue to tick down as Hetty (Linda Hunt) and the team run out of plans to save them.

"Once they learn what these terrorists have planned, [Sam and Callen] realize that the only way they can stop them is to sacrifice their own lives," said executive producer Shane Brennan, according to TV Guide Magazine. The Navy, however, will have its own plans. "If a hostile submarine is sounding off the California shores, the U.S. Navy will sink it. It's that simple! Hetty and Granger realize that ultimately they're powerless, because the Navy is taking independent action."

Callen Gets New Love Interest On 'NCIS: LA', Which Well Known Character Will It Be?

Meanwhile, Kensi (Daniela Ruah) and Deeks (Eric Christian Olsen) will continue the investigation from land before taking to the air. The partner/couple will have their own tension to deal with, however, as the DEA's Talia Del Campo (Mercedes Masohn) continues to muddle in the tenuous Densi relationship.

"These two characters who are obviously very strong women," Ruah explained in a featurette. "Kensi's attracted to Deeks, Talia might also be attracted to him but also sees that this is sensitive ground and uses that to sort of prod and annoy Kensi and bring up subject matter that Kensi does not want to talk about, much less with her."

Scroll through the behind-the-scenes photos above and check out the videos below to see what's in store for "Deep Trouble: Part 2." NCIS: LA air Mondays at 10 p.m. ET on CBS.