The moment Chicago Fire fans have been waiting for finally happened: Casey (Jesse Spencer) and Dawson (Monica Raymund) are married!

On Tuesday's milestone 100th episode, Casey and Dawson tied the knot in an informal courthouse ceremony that was witnessed by their firehouse colleagues and the couple's adopted son, Louie. In a meta twist, the episode also featured characters toasting to the 100th anniversary of Molly's Bar, which finally got a long-awaited financial boost after being added to a local criminology tour.

But it seems like Casey and Dawson's honeymoon period may be cut short. At the end of the episode, Casey confronts a man who appears to be stalking him. The man says he's Louie's father, and he's come to take back his son.

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TVGuide.com caught up with Chicago Fire creator Derek Haas, who explained why now was the right time for #Dawsey to finally tie the knot, revealed how big a threat Louie's father will pose to the newlyweds, and previewed an upcoming love triangle stemming from Severide's (Taylor Kinney) big decision.

Fans have been rooting for a Casey/Dawson wedding for a long time now. Why was now the right time for them to take the plunge?
Haas:
We definitely wanted to do something surprising and special for the 100th episode, and when we found out that ... the winter finale was also going to be the 100th episode, we thought, OK, great. This was in July. We thought, let's just build towards them getting married and we'll throw the audience off where you think that they're not going to, we're not going to do it yet, and then we'll come back from the last commercial and have them get married. So, it was planned since July.

Why was a courthouse wedding the right way to go for them?
Haas:
We were thinking, what can we do different than what we've done? We've done a firehouse wedding for Boden. We did a smaller wedding at Molly's for Platt and Mouch. Then we were thinking, OK, the final call for the 100th episode, we want to have everybody in action, everybody doing something. ... Everybody's dirty and everybody's grimy and everybody's in their turnout gear. And then, they have to go directly to Molly's. The speech for the 100th anniversary is really what we wanted Herrmann to tell the cast and crew and fans of the show. This is how we feel about it. Everybody would be in their turnout gear for that. And then, I'm pretty sure it was [executive producer] Michael [Brandt]'s idea that, what if we just have Casey and Dawson get married in their turnout gear? Because essentially, that's what the show is. The show started with them in their turnout gear, and that would be a cool, different way to do it. So that's what we did.

How big of a problem is Louie's biological father going to be for them going forward?
Haas:
It's definitely going to be a big problem. I think as writers, we always try to surprise first each other and then the audience. So I don't think that story's going to go where you think it's going to go, or where anyone thinks it's going to go. It's going to take some twists and turns and have a surprising conclusion. Typically these kind of stories go over three, four, five episodes and I think that's what you're going to see here.

We see Severide have kind of a crisis of conscience in this episode, and it seems like this bone marrow donation opportunity comes along at the perfect time.
Haas: As firefighters, oftentimes you drop victims off and you never know what happens to them. So we thought, is there a way to combine an emotionally compelling and selfless and romantic story with Severide's time in his life where he's thinking, I should be doing more with my life, beyond just saving people and then never hearing what happened to him?

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The bone marrow donation seems like kind of a life raft for Severide. Will he be a changed man going forward?
Haas:
That's a great way to put it, a life raft. Through the episode, he's looking in mirrors multiple times, very introspectively. Severide isn't that introspective of a character. In fact, in the scene when he's lying in the cot in Casey's office, we wrote in the action description, "Severide being unusually self-reflective," I think were the words. So, I think he's happy that the donation is going to be a painful process, because I think he is trying to say, I need to feel something beyond just going through women and drinking and those kinds of things that he's done over four and a half years.

Clarke warns him that the procedure is going to be painful. Assuming he goes through with it, how much will we see that affect him when the show returns?
Haas:
It is an incredibly painful experience, getting bone marrow drawn from you. In fact, one of our writers, Michael Gilvary, did one of these bone marrow donations. And so, we know how painful it can be, but it's not debilitating. It's a couple of weeks of recovery, and then you'll still be in pain, but you can do your job. so that's how we're going to play it with Severide — if he is a match. Which he is.

After all this, is there a chance for him and Kidd (Miranda Rae Mayo) to get back together?
Haas:
She is going to be integral to that storyline, and we definitely have a triangle coming up with Kidd, Severide and Anna is the name of the woman who's sick in the bed. The actress who plays her is fantastic, so we're looking forward to what the audience's reaction is to that in the second half of the season. But that's something that's going to play out over the whole second two-thirds of the season.

Are you saying she's going to be a love interest for him?
Haas:
Yeah. Even from that little scene at the end. Our audience is kind of in tune, when we've had a guest actress come in. I think Sarah Shahi was one that really connected with Severide. And so, we're hoping for a similar type of situation there.

Severide's relationship with Kidd fizzled pretty quickly at the start of the season. Is that part of what's caused him to reflect more on his life?
Haas:
Yes. That's definitely it, and then I think when he gets to know Anna more in Episodes 9 and 10, and her personality, it definitely fits the Severide type - an independent, strong, loves life kind of a personality that he's attracted to. And that Kidd also has. So it's going to be an interesting triangle.

What else do you guys have planned for the back half of the season?
Haas: We have some bigger episodes coming. We're gonna do a couple of the big type storylines that we've done in the past, where the entire house is affected. ... We're going to have a shocking ending around Episode 13 that's going to lead into 14, where 51 is never going to be the same. ... And there's going to be a call that Casey goes on where he has to decide who he's going to save in a real Sophie's choice. And it's going to be haunting, but it's also going to have ramifications that take up the next episode.

Chicago Fire returns with a two-hour crossover episode on Tuesday, Jan. 3 at 9/8c on NBC.