On Thursday's winter finale of Grey's Anatomy, Bailey (Chandra Wilson) deals with her first major crisis as chief of Grey Sloan Memorial, when nearly 40 firefighters are brought in from a wildfire with severe, life-threatening burns.

But in addition to their case load, the docs have more pressing matters on their minds - namely, figuring out what's behind the (quickly escalating) rivalry between Owen (Kevin McKidd) and Dr. Nathan Riggs (Martin Henderson). Does it have to do with a patient? An ex-girlfriend? Are they half-siblings?

Apparently, none of the above. Owen's mother arrives at the hospital because her boyfriend is one of the critically injured firefighters, and after a conversation between her and Meredith (Ellen Pompeo) - much to a rejected Amelia's (Caterina Scorsone) annoyance - we learn that Owen had a sister. The plot thickens!

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Other points of note in the episode: Arizona (Jessica Capshaw) flirts with a patient (who ends up having a girlfriend); Jackson (Jesse Williams) and April (Sarah Drew) argue about whether they had breakup sex or makeup sex; things seem to be getting more serious between Maggie (Kelly McCreary) and Andrew (Giacomo Gianniotti); and Jo (Camilla Luddington) starts to get jealous of how close Alex (Justin Chambers) and Meredith have become - but then finds out it was only because Meredith was hiding the engagement ring that Alex uses to propose to her. Whoops.

But back to that big Owen reveal. We'll have to wait until the show returns on Feb. 11 to get the details of the Owen and Riggs' rivalry, but in the meantime, TVGuide.com chatted with Martin Henderson to see if we could tease it out of him. Check out our Q&A here:

TVGuide.com: We got a little more info in tonight's episode about Nathan's beef with Owen, but what else can you tell us?

Henderson: There was a historic event that greatly affected both Owen and Nathan's lives, and a tragedy concerning Owen's sister, for which Owen holds Nathan accountable. And they both have very different versions of the same event, hence the fighting and contempt and the resentment. But we're yet to see exactly what the details of that history are.

Does he have a personal or professional connection with Owen's sister?

Henderson: I think it might have been a bit of both.

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Nathan's presence has triggered Owen's PTSD. Does the animosity between them impede Nathan's ability to do his job too?

Henderson: It starts to, yeah. I think at first, given the gravity of the circumstances that they're both dealing with and Owen's perspective on them, Nathan knows going into this that there's no love lost there and that it's going to be awkward between him and Owen. But Nathan also feels very clear about his side of the story and is able to leave that behind him and go ahead and do his job. However, I think Nathan doesn't quite count on Owen being so vocal outside of their relationship with it. So, what starts to happen is, Owen's contempt and resentment starts to poison other people in the hospital against [Nathan], and that starts to make it a lot harder for him to do his job.

For fans, all we have to go on at this point is that Owen hates Nathan. But is Nathan actually a good guy?

Henderson: I think he's a good guy. A lot of the work that he was doing before coming up to Grey Sloan Memorial was over in the Middle East in very war-torn areas, doing a lot of work with relief organizations, giving of his time and his skills to try to help people. So there's a genuine kindness and altruistic aspect to the character. And just on a professional level, he's very, very good at what he does. There's sort of a maverick aspect to him as well. That puts him at odds sometimes, because the way of working that he's learned actually saves lives in the field doesn't necessarily always jell with the protocol of a hospital institution. And that sometimes puts him at loggerheads with other characters as well. In some ways, I think the writers have brought him in to rustle a few feathers and to purposefully be a point of contention.

He seems to be very laid-back and easy-going, and I think there's a very quiet confidence, knowing that he's very good at what he does. He's not out to try to win anybody ever. I don't think he's political in any sense. He's just there to do his job, and he doesn't really care too much. But when other people's opinions of him start to interfere with his ability to do his job, then you start to see him fight back a little bit. There's a bit of fire in him.

You mentioned his military background, which Owen also has. Is that another clue to their connection?

Henderson: It is. In another couple of episodes, we start to learn a little bit more about the fact they do have a shared background, and that at one time - ooo, I'm not allowed to say.

Obviously Nathan has no idea about Amelia's history when they go to the bar. Will her drinking cause more problems for him and Owen? What happens when he finds out he inadvertently caused her to fall off the wagon?

Henderson: Personally, obviously knowing the show and knowing her character, I was deeply upset when I read that. I was like, "No! No, Riggs! Don't give her a drink!" But yeah, he knows nothing about it, and he's ignorant of the fact. But how he reacts when he does find out, I imagine he'd be mortified, as anyone would if you played a hand in someone's loss of sobriety. Whether Owen is privy to that, I don't know. I don't know whether Owen even knows that her and [Nathan] were at the bar at the same time. That's yet to be seen.

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Despite difficulties with some coworkers, Nathan is obviously close with April. Will we see her turn to him for support in the midst of her problems with Jackson?

Henderson: They have a huge history, and obviously going through the challenges of working in the Middle East together, they're very much bonded and I think there's a deep trust between them. So, she does open up to him about what she's going through. It's interesting. From the scenes I've done around that, I can tell he's very neutral about the advice he gives. I think he sees himself more as a sounding board and someone for her to get stuff off her chest, rather than telling her what to do or giving advice. I think there's just a very nice respect there between them too. I think he respects that she's an adult and that she's going to make her own decisions. He certainly doesn't know what would be the right relationship advice. I think it's hard enough to navigate a relationship when you're the one in it, let alone when you're not.

There have been reports that Nathan will be a love interest for Meredith at some point. Can you tell us anything about that?

Henderson: I think with Shondaland, anything's possible. I would never bet money on anything, just given the way they twist and turn things. I think that is maybe an easy assumption, just because of the timing of Derek's departure and then Nathan's arrival on the scene. And the writers may play around with that. They've already played around with that, just by putting our two characters together. But clearly she's still very much mourning that loss, and they don't really have a lot to do with each other at this stage. It's certainly on the [table], but as I said, I think with Shonda, the way she writes and twists things around, anything's possible.

So, Grey's fans, what do you think is the full story behind Owen and Nathan's rivalry? Do you think that drink is the start of a downward spiral for Amelia? See you in February!

Grey's Anatomy returns with new episodes on Feb. 11.

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