NCIS: Los Angeles NCIS: Los Angeles

The end of NCIS: Los Angeles' first season finally offered some clues about Callen's mysterious history. But in the tradition of the hit CBS franchise, every answer spawns three more questions.

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"Everything you saw at the end of last season with Callen [Chris O'Donnell], in one way or another, is resolved," executive producer Shane Brennan tells "But it isn't necessarily resolved in a way that gives you an answer. You get an indication of something that's happened, and then you'll realize we're in for the long haul."

After Callen received some key details about his sister in the finale, the final moments of the episode revealed that someone has been following Callen — perhaps for a very long time. That element is what plays into the "long haul."

"If you want to know what's behind that moment, you're going to have to wait until the end of NCIS: Los Angeles in 10 years time," Brennan says with a laugh. "I've always said that Callen finding himself is the journey this audience will be taken on. The truth of what he finds out is startling. It's a mystery now, and the further he goes down the rabbit hole, the deeper the mystery gets."

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But the premiere has other mysteries to solve, including the sudden disappearance of the team's newest member, Marty Deeks (new series regular Eric Christian Olsen), which leads the team to a human trafficking ring. Deeks ultimately will be partnered with Kensi (Daniela Ruah), a pairing Brennan is particularly excited about.

"In the way that Sam [LL Cool J] and Callen have this great banter, you've got the same thing with Deeks and Kensi," Brennan says. "When you bring characters together, the chemistry will sometimes pop and dictate the direction that you take characters. Other times the audience says, 'You've got to go in that direction.' But I'll tell you there is great chemistry there, and these are two really good-looking characters."

But Olsen and Ruah have differing opinions on whether any romantic sparks will actually fly between the two. "I think Marty is a little bit mesmerized by her," Olsen says. "She's like Jason Bourne, but super hot. She can fly helicopters and shoot rocket launchers and probably beat the crap out of him, and that's incredibly alluring to him."

Check out photos of the NCIS: Los Angeles cast

Ruah, however, believes Kensi's professionalism comes first. "I think she would weigh the ups and downs before making any kind of move," she says. "At the end of the day, they work together. I think it's possible, but it would have to be a really blinding attraction for them to risk their professional relationship."

Callen will get a definite love interest in the form of Marisol Nichols (24, The Gates). And Brennan promises some weighty material for Sam Hanna as well. "We put Sam into some very extreme situations very early in the season," he says. "There's a very powerful episode in the first half-dozen that will take the audience back to Sam as a Navy SEAL. It involves something that's happened to Sam in his past that directly affects what's happening to him now in the course of an investigation."

But the character to watch closest this season is Hetty (Linda Hunt), Brennan says.

"She is a character who is blossoming. If you like Hetty, it going to be, 'Hang on to your seats!'" Brennan teases. "There's a little moment at the very end of Episode 2 that lifts the veil on Hetty. It's setting up something that comes later in the season that will be one of the most powerful episodes we do this season."

NCIS: Los Angeles returns with back-to-back episodes Tuesday at 9/8c on CBS.