Husbands (we love writing this as much as you love reading it) TK (Ronen Rubinstein) and Carlos (Rafael L. Silva) are returning for the fifth and final season of 9-1-1: Lone Star (premieres Monday, September 23 on Fox) is already dealing with the unsolved murder of his father. But there’s more drama to come.

Carlos’ obsession with this investigation will test their marriage. TK’s half-brother Jonah is coming to Austin (which is sure to impact Rob Lowe’s Owen, too). Below, co-showrunner Rashad Raisani gives a sneak peek at that and more to come for the fan-favorite couple. Plus, above is an exclusive sneak peek at Carlos and TK in the Season 5 premiere.

TK’s half-brother is coming. What can you say about what brings him to Austin and what that means for TK and Owen?

Rashad Raisani: Good question, because it’s not just about TK, it’s about Owen. We had such a big storyline in Season 2 where Owen’s ex Gwyn (Lisa Edelstein), TK’s mother, got pregnant and Owen thought it was his. Then it turns out it was actually Enzo, her second husband and the guy who took Owen’s place in TK’s life after Owen took a back seat as a father because of 9/11. Enzo really had a big part in raising TK and he was the father of Jonah, TK’s little brother. So as we kind of wrap up this series and talk about family, we wanted to get back to who TK is going to be as a son and as a brother and also as a father when this series ends. Jonah, this little boy, and Enzo, his stepdad, seemed like the perfect characters that we could kind of tell this story through.

Also, Owen will be forced to acknowledge that TK was damaged by his past as TK’s father and the years he was away, and that Enzo had a lot to say about what kind of person TK would become. So there’s a lot of reckoning about the past in his character. Plus, he’s just a lot of fun. So I think there’s going to be a lot of fun to come out of that as well.

Carlos is investigating his father’s death. What can you tell us about this investigation? Campbell (Parker Young) will be involved too, right?

We’ve talked about Campbell and Carlos because they’re like two brothers, the big brother and the little brother. Carlos comes in and in Campbell’s eyes he’s kind of been given a free pass because his dad, Gabriel Reyes, was a legend and he was able to be right up there and be a Texas Ranger. The first episode is about these two brothers fighting for dad’s love and dad is Chief Bridges, their boss. But what’s going to happen is they’re going to learn to respect each other and they’re going to discover that they’re so much better when they work as a team and they really complement each other brilliantly, not just as investigators but as characters and, for me, frankly, as actors. I think they just have a wonderful dynamic that gets better and better as the season goes on.

The murder of Carlos’ father is one thing that Carlos won’t give up on. It’s the central driving force in Carlos’ life. And there’s no way we’re ending the series without giving this poor guy some answers. So at the end of the day, there will be some answers.

The season synopsis says that TK and Carlos’ marriage will be tested because Carlos is obsessed with solving his father’s murder. What can you tell us about how that plays out and how difficult it will be for them? I’m confident they’ll be OK in the end.

An important aspect of their marriage for me was that I wanted to treat it like a real marriage. What I mean by that is that no matter who you are, even if you are so wonderful together and so good-looking, life is not perfect. Everyone has real problems that affect even great relationships. And the real strength of a relationship is not the absence of challenges and problems, but how you deal with and face them, and whether they strengthen or weaken the relationship.

Kevin Estrada / FOX

And so Carlos is going to have the weight of his father’s murder on his mind and it’s going to take up a lot of the oxygen in the room, and TK, who is an incredibly supportive husband, is going to try to be patient and graceful and give Carlos that space. But at a certain point, he’s going to have to commit to them having a future together, and that’s part of what you talked about with Jonah. I like to think that in some ways, this little boy represents the future of their relationship and Gabriel’s murder represents the past of their relationship, and there’s going to be a tension between those two things, and that’s going to be their central dynamic this year: that one person is struggling to find answers in the past and the other person is struggling to move on and build a future, which I think is a very relatable human experience.

All this makes me think about children, and for them this has been somewhat of a problem in the past. What role does this play?

I think that’s another pressure point because Carlos has been very open with TK about, “Hey, I’m not ready to have kids and I don’t know if I ever will be,” and now that his father has been murdered, in his mind, that’s the last thing he has the bandwidth or emotional energy to deal with. But like I said, TK feels like life is happening right now, like we don’t get moments back, and especially as first responders, they see that you can’t take anything for granted. And circumstances are going to arise that lead to a situation where TK says, I know you were uncomfortable as a father taking this next step, but I feel like it’s something powerful that I need to do, and I need to make this a crisis point for them as well.

I hope that the audience can understand the argument from both perspectives and just say, “Shit, I don’t know what to do. I don’t know what I would do.” I hope so, because I feel like people have real problems where you can’t just say, “He’s right, he’s wrong, whatever.” We just wanted to throw that at them.

9-1-1: Lone StarPremiere of the fifth and final season, Monday, September 23, 8/7c, Fox