Since the start of True Detective, we’ve known that something big happened in the year 2002—something that tore apart the investigative partnership of Rust Cohle (Matthew McConaughey) and Martin Hart (Woody Harrelson). But we never knew what it was.

Until Sunday night.

(Warning: stop reading now if you want to avoid spoilers.)

The title of Episode 6 of the hypnotic HBO series—“Haunted Houses”—was fitting: this was the episode in which we finally discovered that it hasn’t just been a gruesome murder that’s been haunting Rust and Marty for the past decade or so.

Turns out a domestic drama has been haunting them, too—and Marty’s wife, Maggie, is at the center of it. When Maggie discovers that seven years after his last infidelity, Marty is now cheating on her again, she decides to do something drastic. First she visits a cocktail bar and strikes up a conversation with a stranger. But that doesn’t seem devastating enough. So in the middle of the night she shows up at Rust’s door with a bottle of wine in her hand. One thing leads to another. Minutes later, a regretful Rust is shouting at Maggie to “get the fuck out of here,” and Maggie is heading home to tell Marty what she’s done.

Much has been said already about McConaughey’s mind-blowing performance, and the sex scene at the heart of “Haunted Houses” is no exception. But the real revelation Sunday night was Michelle Monaghan (Mission Impossible, Gone Baby Gone), the gifted Iowan who plays Maggie. Monaghan has been on the sidelines for much of the series, but now she’s center stage; divorced from Marty, she’s even become part of the 2012 investigation into the “copycat” killings that happened after the initial murder 17 years earlier. She brings a steely, earthy sensitivity to the role—a welcome contrast to Harrelson’s callous machismo and McConaughey’s mystical nihilism.

To delve deeper into Maggie’s psyche, we gave Monaghan a call. She explained why Maggie is attracted to Rust; why sex with him was a way of setting herself free; and how Marty’s family is going to factor into the remaining two episodes. Excerpts from our conversation:

Critics have said that True Detective is male-centric, that it’s the Rust & Marty show, and that female characters have been marginalized. What’s your response?

I think Episode 6 is a good rebuttal to that critique. The reason why I signed on to the project is because I loved Maggie. I thought she was a completely fleshed out character. She has this very meaningful arc. She starts out as a nurturing person—a very protective wife and mother. She’s trying to keep her marriage intact. But ultimately, in Episode 6, after Maggie finds out about Marty’s infidelity once again, and she sees her own daughter’s behavior unraveling, no doubt because of Marty’s absence, she realizes she really needs to do something critical in order to save herself and her daughters. And the ultimate revenge for her is to have an affair with the person Marty is most threatened by—his partner. She knows that Marty will never, ever be able to recover from that.

I’m curious about Maggie’s motivation for going to Rust’s apartment. You used the word “revenge.” Was it revenge? Or was she trying to free herself?

Oh, I think she was freeing herself. Definitely. There’s some revenge there, certainly, and I think you see it when she sits down at the kitchen table and really sticks it to Marty. But the ultimate choice—the choice to sleep with Rust—was made to free herself from the marriage.

The moment when Maggie comes to Rust’s apartment—there’s been an attraction there since the beginning. Was it always sexual?

Maggie has this really engaging relationship with Rust—a friendly relationship—at the same time she almost has a non-relationship with her husband. And I think that’s the attraction she has to Rust. From the moment they sit down to supper together in the first episode, she sees somebody who’s a little awkward, a little uncomfortable, a little vulnerable. She’s a very curious and inquisitive person, and when Marty leaves the table she takes the opportunity to ask his new partner the questions she’s been dying to ask, and Rust answers them truthfully. And I think that shocks her, because Rust engages her in a way that Marty doesn’t. He seems to be open with her. He enjoys spending time with her. And she starts to sincerely care about his well-being. She becomes a matchmaker for him. She wants him to look after himself.

I thought the sex scene in Sunday’s episode was remarkable—such a complex and believable mix of lust and anger and regret. You and Matthew McConaughey nailed it.

Thank you! When Maggie takes advantage of Rust, it hurts her as much as it hurts him. She’s not proud of herself. That is the most devastating thing for her, ultimately—that by being selfish, by using him, she hurt this person she really cared about.

When the police ask Maggie whether her divorce had anything to do with Rust, she lies and says no. Why?

Once a cop’s wife, always a cop’s wife. She knows the game. She doesn’t trust Marty, but she definitely doesn’t trust these guys. She’s a very smart woman. And no matter what, she has integrity. She’s not going to throw anybody under the bus.

You've said that shooting True Detective was like making a long movie. Do you see True Detective as a tipping point—a show that will encourage more actors, directors, and writers who've focused primarily on movies to try their hand at TV?

I definitely do. Especially in terms of this format—eight, 10, 12 episodes and then you’re done. Beginning, middle, and end. It’s not a huge commitment. So you can get a great caliber of talent. And HBO leaves you to it. They put complete trust in the creative team. So I think we’ll see more of this. I’d love to be a part of something like this again.

Creator and writer Nic Pizzolatto was on set the whole time, Cary Fukunaga directed every episode, and you could sit down with them and talk through Maggie’s motivations. What’s an example of a scene where that dynamic was really crucial for you?

Definitely my interrogation scenes. You don’t see Maggie for a while before the interrogation, so obviously a lot of time has passed. I really had to grapple with some big questions. Where is Maggie in her life right now? Has she forgiven Marty? Why isn’t she throwing him under the bus? And that’s honestly one of the things that appealed to me about True Detective—the fact that it spans 20 years. I’ve never done anything like that before. I was so interested in not just physically aging, but emotionally aging—seeing where Maggie goes emotionally. That’s why it was so important to have Nic there with us, to get inside his mind. He knew exactly who these people were in each point in their lives.

That’s a great point about “emotionally aging.” I’m so glad Episode 6 included Maggie in that process—not just Rust and Marty. What are we learning about Maggie now that we didn’t know before?

I think that you’ll see that she’s really made a big change in her life, in terms of the people who are a part of her life now. She moves on into a new relationship. She’s always been the bigger person. That hasn’t changed. She still very much cares about Marty, but she’s let go of the past.

There’s a line I loved in this episode. Maggie quotes Rust as saying “There’s no such thing as forgiveness. People just have short memories.”

I think Maggie kind of agrees. People move on. It’s not so much about forgiveness. It’s about time—it’s time that heals. For her, at least. She’s pretty evolved. She can forgive. I think she’s let go of Marty’s infidelities, as you’ll start to see in future episodes.

Still, Maggie has been hurt. One of the things Nic told me when I interviewed him was that True Detective is in part about the damage men do to women and children.

Definitely. Look at any part of the series and you’ll see there are a lot of lost female souls. You ask yourself, “Why are women taken advantage of? Why are they lost?” And then you see it reflected in the Hart family. You see Marty’s absence. Even though he’s out investigating people who have victimized women and children, he’s making his wife and children suffer.

Now that Maggie and Marty are divorced, is Marty’s family still going to be part of the plot going forward?

You mean my family? As in, my mother and father and that sort of thing?

Sure. I was wondering mainly about your daughters, Audrey and Maisie, but Marty’s father-in-law—Maggie’s father—did show up earlier in the series, living in a big waterfront house and complaining about “kids these days.” Let’s put them all on the table.

Yes, yes. Our family—everybody—is still going to be part of the plot going forward.

Have you followed any of these speculations online about the supernatural elements of the show?

No, I haven’t. But now I’m completely intrigued!

It’s basically about how the mythology of these serial killers is tied to a collection of horror stories from 1895 called The King in Yellow. Did you ever puzzle over this stuff when you were reading the scripts?

Oh, I am fascinated by Nic. I’m fascinated by his brain. He did an incredible amount of research and work. He’s extraordinarily well-read and aware of a number of different things that I have no clue about whatsoever. [Laughs.] I remember there was this picture he saw somewhere that inspired the end of Episode 3—you know, when we see Reggie Ledoux in his underwear, wearing that gas mask. The original image is out there somewhere.

So you haven’t downloaded the complete works of Robert Chambers? I’m curious whether everyone on set was indoctrinated in the mythology of Carcosa and the Yellow King—or if Nic left those themes on the periphery.

I think it was on the periphery. But Woody and Matthew might say differently!

OK, final question: if you had to pick one word to describe how Season 1 of True Detective ends, what would it be?

Staggering.