‘Slow Horses’ Boss Breaks Down Shocking Finale Ending

Meredith Jacobs Wed, October 21, 2026 at 10:28 AM UTC

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Jack Lowden — 'Slow Horses' Season 6 Finale "Judgment Day" - Apple TV

Slow Horses wraps up its sixth season, based on two books from Mick Herron‘s series, with a finale that kills off a major character and leaves another in dire straits. TV Insider spoke with coexecutive producer Gaby Chiappe about those moments and more. Warning: Spoilers for the Slow Horses Season 6 finale ahead!

The fight between the slow horses and the assassins after them — including River’s (Jack Lowden) father Frank Harkness (Hugo Weaving) — reaches a climax at Diana’s (Kristin Scott Thomas) house where the former are hiding out. And in the resulting gunfight, Flyte (Ruth Bradley) is killed — her death is different from the book — Louisa (Rosalind Eleazar) is shot, and River and Frank sit down for a chat.

Then, in the final moments of the season, after River has returned to his grandfather’s house after realizing that Sid (Olivia Cooke) is there, he realizes he’s been poisoned by opening a bottle of wine … courtesy of the assassins who had been in the home earlier in the season.

Below, Gaby Chiappe breaks down that finale. (Plus, see what the cast had to say about the finale here.)

You can confirm that River’s not dead, right? We know he’s been poisoned, it seems, but he’s not dead dead at that moment.

Gaby Chiappe: We don’t know what’s going to happen to him. We know that Novichok has been named, and we know that it has been used within the episode, and we suspect that that’s what it is. And people survive it, but people also die from it. So we don’t know what’s going to happen.

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River’s going into that house feeling hopeful about him and Sid, it seems like, but at the same time, it’s like, what kind of relationship could they have? Is he even thinking about that? Or is it kind of just like, “OK, she’s here, great.”

She basically says, “I couldn’t think of anybody to go to.” And he’s clearly got feelings for her, so I’m not sure that he’s thinking beyond she’s alive, and I know where she is, because when he finds out where she is from something, a message that Catherine [Saskia Reeves] gives him, he’s out of there at the speed of light. So he goes from thinking he’s lost everything and everyone to thinking Sid is there and I know where she is, and I’m going there. I’m not sure how far ahead he’s planning that relationship, but he’s also getting stuff ready when he realizes that he’s been poisoned. And he’s thinking, is it a cup of tea or a glass of wine? What’s more appropriate? He’s clearly thinking relationship.

But we also know that Sid is in the house. What can you say about what we should be thinking about? Should we be worried about her? Should we be wondering if she’s still there? Is there anything you can say about Sid in that final sequence?

You can hear that Sid’s having a bath upstairs. You can hear the water running. So at some point Sid is going to come down. Is she going to come down soon enough to be able to help River? And you have to be really careful touching. It’s a nerve agent that I think permeates your skin. So she’s potentially in danger, but she’s also potentially the reason that — if River does survive, she could be part of the reason that he does survive.

What excites you about where Slow Horses goes next after that finale ending?

What really excites me about it is that although I’ve read the book, I haven’t yet read the script for the next season because I’m not involved, and I am really, really excited about finding out what’s next. So I’m now back with all — I’m on the same status as any other viewer for Season 7, any other viewer who’s read the book.

Talk about killing off Flyte and having Harkness be the one to do it because that is different from the book.

Yeah. Because we were melding, as you know, we were adapting two books and trying to weave them together to make one book, there are some things that we did have to change, and it felt like the most satisfying and conclusive thing that it would be Frank who killed her. She loses her life in the same way that she does in the original, in that she’s protecting other people. So the heroism of that moment isn’t lost and it feels more weighty where everybody was on the chessboard at the end. It felt the most weighty thing would be Frank to kill her. And also Frank is, just in case you were feeling sentimental about Frank … he really is not a nice person.

Everything he does is like, yes, this is River’s father, so usually it’s like a family member of one of the main characters, there’s going to be something about him you’re going to love. And so it’s just like, no, no, he’s just going to keep being worse.

He’s just awful. I mean, the thing is you love… Well, I do anyway. He’s so entertaining on screen and he’s funny and all the things that all the characters in Slow Horses are, but you can’t forget for a minute that he doesn’t have a good bone in his body. I mean, this is somebody who bred his own children as soldiers.

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I will say I love the moment that Flyte throws her gun at him because that’s just quintessential Slow Horses because obviously it’s not going to do anything, but it’s like she’s not going to give up, but it’s funny, too.

Yeah. I think that the borderline in Slow Horses of comedy and tragedy is, it’s a razor blade between them. And there are often moments which are both tragic and funny.

River and Harkness’ conversation is so good and so indicative of the only type of relationship that this father and son can have. What had you wanted to do with that? Because it’s one of my favorite scenes of the season.

Oh, I’m so glad. I felt very lucky to be able — because that’s in my episode, I felt very lucky to be able to write that. I think everybody — Hugo and the director, Adam [Randall], and Jack and Hugo and me — it felt like this is the… They had a conversation in Season 3 in King’s Cross, and that was very satisfying that there was a bit of… You just got some sense of who — they had a couple, but who they might be, that even though there’s no relationship, there’s a sort of reaching for something. There’s a trying to understand what that relationship could have been if it hadn’t been this. And I think River needs to say — his relationship with his mom is dysfunctional, to say the least. His grandfather, who he loved, is dead. Sid has gone at this point. He’s got no one. And just trying to imagine that things might’ve been different, I think is, even if it’s only for a second, just imagining what it might’ve been like to have a normal father who wasn’t a killer, paid killer, it’s very sad. And I think Frank has a moment of, he doesn’t kill River. That’s always good.

I even like the reasoning for him killing Jane. It’s like, oh, she had the gun. It wasn’t even just like, “Oh, I’m protecting my son.” It was just like…

No, no, it’s practical. But you don’t know. He gives the practical reason, and maybe it’s that. Maybe there’s more to it, but I don’t know. I’m not sure that Frank knows what he’s doing.

I was going to ask, is there any part of Frank that thinks he could be the kind of father that would at least care if his son is alive or dead?

I don’t think Frank thinks he cares, but I think it’s indicative that in Episode 4, when he’s talking to Jackson Lamb [Gary Oldman] in the Shake Shack and finds out that people have been killing slow horses, pretty much the first thing he says is, “Did they get anyone I know?” So there’s something about River’s existence in the world — and all his other sons are dead. The ones he bred to be super soldiers, they’re all dead. So River is the only thing left. And I think he can’t help but acknowledge that in some part of him.

Technically, River lets Harkness go, though he wasn’t really in a state to try to stop him. Still, why and where is River when it comes to his dad after he’s walked away at the end there?

I think he’s really conflicted, really, really conflicted. But not only is River not in a state to chase him, Frank is really not in a state to run. So yeah, he lets him go, but how far is Frank going to get? Again, it’s another we don’t know. And I think there’s a sort of sense of whatever at the end of that scene. It’s kind of like, he knows he’s not controlling anything. You’d have to ask Jack what he was feeling, but I think you let Frank go, but where’s Frank going to go really? He’s mashed up.

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That is true. Roddy (Christopher Chung) returns to help Reece (Lenny Rush) and Shirley (Aimee-Ffion Edwards). Does that bravery surprise even himself?

I think Roddy for a moment there is the person that Roddy would really love to be and probably knows he’s not just occasionally, but he’s so delusional, Roddy. Yeah, he does the brave thing, and he’s the person that he wished he was, but also that Flyte needed him to be. Flyte in that episode needs him to step up to help, to help Reece, and just to be someone she can rely on to do the right thing when she needs people to be doing the right thing. So I think it’s very moving that he’s inspired to turn back. It doesn’t look like he’s running away. He’s inspired to turn back when he sees her dead and goes back into the jaws of death, shooting a gun that he misses pretty much with every bullet.

Yeah, but that’s Roddy. So, Diana manages to save face and her job, unsurprisingly. What is it about her that she’s able to do that like she does?

She’s very clever, and she’s totally shameless, and I think she’s been in that world for so long that she knows every lever to pull, and she will just — at one point in this episode, she is prepared to throw herself on her sword because she can see no other way out. But once she’s shown that there is a way out, she doesn’t let go of it. She’s the arch manipulator and the arch machinator, and she just keeps going until there is absolutely no way out. That way out has to narrow to a tiny pinpoint before she’ll give up on it. She’ll just keep thinking and scheming.

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