[SPOILER ALERT: The following article contains information about the upcoming season of 'NCIS: Los Angeles']

LL Cool J may have participated in CBS's salute to the ­Beatles earlier this year, but he certainly won't be ­singing "We all live in a yellow submarine" when his Sam and Chris O'Donnell's Callen find themselves trapped in a hostile submersible as "NCIS: Los Angeles" opens its sixth ­season. Last May's cliffhanger found the partners taking an unexpected trip beneath the Pacific Ocean in a narco sub — that is, a submarine used to smuggle drugs out of Mexico — which now may be carrying substances even more illicit than anything you can shoot up or snort.

"Once they learn what these terrorists have planned, they realize that the only way they can stop them is to sacrifice their own lives," says executive producer Shane Brennan. Or have their lives sacrificed for them, since, Brennan adds, "If a hostile submarine is sounding off California shores, the U.S. Navy will sink it. It's that simple! Hetty [Linda Hunt] and Granger [Miguel Ferrer] realize that ultimately they're powerless, because the Navy is taking independent action."

But salvation just might come from above when Kensi (Daniela Ruah) and Deeks (Eric Christian Olsen) take a helicopter to search for them. The involvement of those two guarantees some lighter moments in an episode that otherwise takes place in tense real time. Adding to the tension: DEA agent Talia (Mercedes Masohn) reappears to briefly reanimate their love triangle. "Talia gives Kensi some advice that she doesn't like," Brennan teases, "and things get heated."

How hot, or cold, will the "Densi" interaction be this season, now that the Bickersons have finally hooked up? Surprisingly, perhaps, the reset button on that romance hasn't been hit. "There are a lot of fun moments between them early in the season as they try to sort out their relationship," Brennan reveals. "It isn't as black-and-white as people think, and we try to keep it as a real workplace romance, especially one with guns and explosions and people trying to kill you. We don't want to leave the audience thinking that now we've turned it off and we might turn it on again later. We're trying to integrate the ups and downs of their relationship throughout the season." Expect downs early on. Ruah tweeted a photo of herself in boxing regalia, which could offer a clue to the state of their union. "There's a very intense fight scene with her and Deeks in one episode, which is a lot of fun," Brennan says. "Kensi could be taking a little bit of her frustration out on Deeks, so it's lucky they're both wearing head protection."

If you wondered whether or not Callen might ever see some action, too, well, you may not have to be envious indefinitely, O'Donnell fans. Brennan promises that the new season will bring "more than a little romance," for Callen, and he's not just making that vow to further tease "Densi" shippers. Later in the season, Callen will be involved in a fairly intense romantic arc "featuring one of the characters the fans have come to love as well," Brennan says, before adding, "I won't say who it is and I won't say anymore about it."

There will, however, be no romance for Hetty, just a contentious Senate hearing. Her own professional problems take a backseat in the season opener, but once the boys in the sub presumably shake off their sea legs, Episodes 2 through 4 will focus largely on the boss's travails in the wake of her offered resignation letter. Even elected officials are questioning the operation that took Kensi to Afghanistan last year, and, Brennan says, "it reaches a point where Hetty, who knows everyone and can get things done, cannot talk her way out of this — in effect, she runs out of friends. And at the same time, we learn that someone is out to get her. It really tests all of them and particularly tests Granger." Also at the administrative level: The long lag time between crossovers with the original "NCIS" series will finally come to an end as Director Vance (Rocky Carroll) returns for several substantive appearances this season.

Because of the extended storyline the producers came up with last year to accommodate Ruah's pregnancy, "we're lucky this season," Brennan notes. "We have a lot of stories we held over from last season, thanks to the Kensi-in-Afghanistan arc. This season, because we don't have that prolonged storyline, everyone's excited about some of the smaller arcs we're doing and some of the really intense action episodes we've got planned."

That doesn't mean they want to use leftover ideas at the expense of ripping new ones from the headlines. "The sad reality is, the international news often involves the U.S. in one way or another," Brennan says. "Our stories sometimes reflect what's happening in the world, and because the world has probably become a more dang­erous place in the last four months than it has been for the whole series run so far, there's a lot of material out there. And we may well be going into the bear's den" — by which, he means, specifically, Russia. Just probably by air, not by sea, if Callen and Sam have anything to say about it.

"NCIS: Los Angeles" returns Monday, Sept. 29 on CBS.

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