SPOILER ALERT: Do not read ahead if you have not seen the Nov. 19 episode of “Grey’s Anatomy,” titled “Things We Lost in the Fire.”

“Grey’s Anatomy’s” new leading man Martin Henderson has ruffled many feathers since he walked into Grey Sloan Memorial Hospital as Dr. Nathan Riggs.

Butting heads with Owen (Kevin McKidd), the entire medical staff — and plenty of viewers — have been wondering what went on in the past between Owen and Nathan, and Thursday night’s midseason finale finally shed more light on the dueling duo’s history.

A big fire brought tons of injured firefighters into the hospital, marking Bailey’s (Chandra Wilson) first huge crisis since assuming her role as chief. Among the firefighters was Owen’s mother’s boyfriend, who ended up being treated by Nathan, much to Owen’s dismay. With Owen mad that his enemy was performing surgery on his mother’s man, Meredith (Ellen Pompeo), along with the rest of the MDs, was overly curious about Hunt and Riggs’ relationship — so Mer ended up talking to Mama Hunt to get some insight, which then stirred up a major fight between Dr. Grey and Amelia (Caterina Scorsone) who said she’s not being loyal to her — ending with Mer telling Derek’s sister that she wants her to move out of her house.

Later in the episode, Amelia ended up going to the bar with Riggs after her fight with Mer, and although she declined a beverage from the new doc, the former alcoholic ended up ordering a drink for herself.

Another duo fighting during the midseason finale were Jo (Camilla Luddington) and Alex (Justin Chambers). Their bickering couldn’t even be resolved by a proposal — Karev popped the question, and we were left with a major cliffhanger: “Yes or no?” he asked, to no response.

Back with the boys, Owen and Nathan got into a physical fight, which resulted in Nathan getting punched. But why?

The reason behind Owen and Nathan’s bad blood isn’t perfectly clear at this point, but throughout the episode, Nathan called Owen’s mother “Ma,” and at episode’s end, Meredith told Owen she didn’t know he had a sister.

While all signs point to Nathan being romantically involved with Owen’s sister — which seemingly ended up going terribly wrong — Henderson spoke to Variety to clear up Nathan’s storyline.

Based on tonight’s episode, we have to assume that Nathan was heavily romantically involved with Owen’s sister, and something went terribly wrong. Is this a fair theory?

There’s a reason why you’re a journalist — your instincts are impeccable! Yes, I think you’re on to something. [Laughs.] In typical “Grey’s” fashion, the mystery is sort of going to build very slowly throughout the episodes. At this point, we don’t know much more, except that there was some serious involvement between Riggs and Owen’s sister, but whether it was marriage or engaged or dating, it still remains unclear. It seems there was a rather dramatic event in their shared history in which both have stark differences of opinion and perspective, and Owen feels that Riggs is deplorable and Riggs feels completely vindicated, given his perspective on the event. More will come to light. Things get more and more aggressive, and I think at some point, the truth will come blurting out.

Owen really, really seems to hate Nathan. Are the feelings mutual?

I think Riggs doesn’t feel the animosity toward Hunt as much as Hunt does toward him, however, with Hunt’s insistence with not only continuing to abuse him physically and verbally, he’s also starting to turn other people in the hospital against him and I think that’s where Riggs is starting to feel more contempt for Hunt.

Is there any chance for these guys to repair their relationship? They are going to be working together, after all, and you are a series regular…

With Shonda’s work, forgiveness is a big part and obviously we look at a lot of healing between the characters. What this show deals with is what it means to be human and with being human, we all make mistakes, so forgiveness is a part of getting on and having relationships. But what the journey that they have to take to get there is still very much a mystery. But I could be absolutely wrong — that’s just my wishful thinking and my romantic ideal. They may end up being butting heads for the rest of my tenure!

When you were first cast, there were many rumors that you’d be replacing Patrick Dempsey, in regards to being Ellen Pompeo’s love interest. At this point, Nathan has flirted with Meredith and Maggie — it seems inevitable that he’ll have a love interest, correct?

The way Shondaland works, there’s probably a lot of truth to that. I don’t think there’s any way to predict it. I’m as curious as anyone to find out who I’m going to be smooching anytime soon. I’m very excited to read that scene, but as of yet, there’s been no indication other than a few little moments here and there of flirtation and whatnot, but there’s nothing that’s actually created any traction. When we do find out, I think it’s going to be fun to see how that relationship evolves, if in fact that happens…maybe he never ties himself down because I know very little about his history romantically, so it’s all a mystery at the moment. But I’ve got a lot of hotties to pick from.

What was your reaction to all of the attention to your casting, especially those reports that said you were replacing Dempsey (whose character Derek Shepherd was killed off), shortly before you joined the show?

I don’t think you can replace Derek Shepherd. I don’t think you ever will. I think what that character was and what Patrick brought to it is unique and what he does, he does brilliantly. I think from the get-go, I personally never compared myself to Patrick or considered comparing my character to his character because they’re so innately different. When I first read reports, I thought that it was really just a coincidence of timing. The pressure I felt was more about coming onto a show and cast and crew that had been working together for so long. I think that was what was daunting. I was definitely nervous on my first day. It was like being the new kid in school, but everyone put my fears at ease immediately. The crew and cast were so welcoming and kind and supportive, and that made me feel at home instantly.

Were you a fan of “Grey’s Anatomy” before you were cast?

I was. I’ve been a fan of “Grey’s” since it’s early stuff. To be honest, I was not a regular watcher of the show — which is not saying anything because I don’t actually really watch any television. When Shonda asked me to come onto the show, I went to watch some of the latter seasons to see how the show had evolved and clearly it has…it’s changed a lot, but what still appealed to me is that the show in one scene can be deeply moving and tragic and dramatic, and the next can be comedic and funny and goofy, and the next can be really poignant…that sort of multi-dimensional aspect is what made me really excited to be involved.

“Grey’s Anatomy” has made famous nicknames in McDreamy and McSteamy. What’s your character’s name going to be?

I’ve heard McKiwi thrown around here and there. I saw a report the other day someone had written something about that, which is fine with me. I love representing my country and where I’m from so if that one stuck, I’d be very happy. McSteamy, McDreamy…Creamy? No, that doesn’t really work.