Nathan Fillion and Stana Katic, <i>Castle</i> Nathan Fillion and Stana Katic, Castle

In the opening moments of Castle's Season 3 premiere, crime-solving partners and star-crossed lovers Castle and Beckett have their guns pointed at one another.

We knew there'd be some tension after Castle (Nathan Fillion) left Beckett (Stana Katic) to run off to the Hamptons with his ex-wife (Monet Mazur) at the end of last season. But this?

While everything may not be as dire as it first seems, there is definitely some bad blood to be worked out.

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"She's 16 now, and that's the time the hormones start kicking in and telling us it's time to rebel, it's time to push the envelope, it's time to break the rules," Fillion says. "We'll see that teenage angst, or at least Alexis' version of that angst, because she's a pretty well-balanced kid."

And whatever embers of office romance still burn come November sweeps will momentarily be doused as Castle and Becket focus on a new foe: a serial killer who murders in threes.

"We have a villain for Castle and Beckett who can be in the wind and continue to complicate their lives and investigations," new executive producer David Amman says. "He's pretty dark, but not to the point that it's a violation of the bounds of the show. When he's in the interrogation room with Castle and Beckett, he's a force to be reckoned with — inside that room and out."

Still, Katic thinks now might be the perfect time for Castle and Beckett to give a relationship a try. "I think they make sense for each other," she says. "I'm the only one on the entire team who's like, 'So what if they get together?' ... Maybe I'm overly romantic thinking that when people get together, they should still have that spice and fire. They should still have the same swordplay."

Castle premieres Monday at 10/9c on ABC.

(Additional reporting by Natalie Abrams)