Slow Horses Season 4 review – Spook street gets personal

🐴 Introduction

🐴 This review is part of our Slow Horses hub — every season reviewed and ranked, plus why the show works so well and what is coming in Season 6.

The first three seasons of this show are about institutions. Careers ruined by them, secrets buried inside them, ambitions pursued through them. Spook Street is about a family, and it is significantly harder to watch.

It opens with a bomb in a London shopping centre. The show handles it the way it handles everything: no slow-motion, no swelling score, just an ordinary Saturday and then a great deal of noise and dust. It is the most disturbing sequence the series has staged, and it is over in minutes, because the season is not actually about terrorism.

It is about David Cartwright, River’s grandfather, the retired spymaster played by Jonathan Pryce. David’s memory is going. He knows it, some of the time. And a man whose entire professional life consisted of holding secrets is now a man who cannot reliably tell you what day it is — which makes him, in the cold arithmetic of the service he gave his life to, a problem to be managed.

For anyone whose own parents are getting older, this season lands somewhere the previous three did not go near.

🧠 Story & themes

The plot mechanism is elegant. David, alone in his cottage, shoots a man in his bathroom. He is certain the man was sent to kill him. He is also certain about several things that are not true. River arrives to find his grandfather holding a gun and a corpse, and has to decide, in about ninety seconds, whether he is protecting a target or covering for a confused old man who has just committed murder.

River’s solution to that problem is the boldest thing the character has ever done, and it removes him from the board for a large part of the season. What he finds while off the books is a place in rural France called Les Arbres and a man called Frank Harkness, and the discovery that his own existence is closer to the operation than he would like.

The theme is inheritance, in the most literal sense the genre allows. Frank Harkness’s programme treats children as an engineering problem: raise them correctly and you get reliable operators. David Cartwright’s version was gentler and, the season argues, not categorically different — he shaped a grandson into a spy by telling him the right stories at the right age. River has spent four seasons desperate to be worthy of a career he never actually chose.

Set against that, the institutional stuff proceeds as usual. There is a new First Desk, Claude Whelan, played by James Callis, who is decent, well-intentioned and entirely out of his depth, and a new Head of the Dogs, Emma Flyte, played by Ruth Bradley, who is the first genuinely competent officer the show has put opposite Lamb in a while. Diana Taverner, denied the top job again, is at her most dangerous.

🎭 Characters & performances

Jonathan Pryce has been a background presence for three seasons — the legend, the granddad, the man in the good armchair. Season 4 hands him the story, and he is extraordinary. He plays dementia without a single actorly tremor, as a series of small confident statements that do not connect to each other. The terrible part is how much of David is still in there: the tradecraft instincts fire perfectly while the context is gone, so he is at his most lethal precisely when he understands least.

Hugo Weaving is the villain this show has needed. Frank Harkness is not theatrical. He is warm, reasonable and utterly certain, a man who has built something monstrous out of entirely logical steps and would be delighted to explain the logic over a coffee. Weaving underplays him to the point of unease, and his scenes with Jack Lowden carry a paternal quality that is the most upsetting thing in the season.

The bench work is strong. Tom Brooke’s J.K. Coe, headphones on, barely speaking, is a superbly odd addition. Joanna Scanlan arrives to inject bureaucratic irritation into an office that had settled into its own rhythms. Ruth Bradley’s Emma Flyte is a proper adversary rather than an obstacle.

And then there is Marcus Longridge. Kadiff Kirwan has spent three seasons making a gambler’s self-destruction quietly funny, and the finale kills him defending the building and the people in it. The show does not give him a speech. What it gives him instead is Lamb, afterwards, fighting Diana Taverner over the size of the payment to Marcus’s family, which is the only way this man has ever been able to say that he cared.

🎬 Craft: The bleakest the show has looked

Adam Randall directed all six episodes, and the visual register drops several degrees. Where Season 3 was propulsive, this one is heavy. The French sequences bring genuine open-air menace into a series that has mostly lived in London drizzle, and the Les Arbres material has an unpleasant, sunlit calm.

The sound design deserves particular mention. The opening bombing is constructed almost entirely out of what a bomb does to hearing rather than what it does to a building, and the season keeps returning to that trick — muffled rooms, ringing, dialogue you have to lean toward.

The one structural weakness is the middle. With River off the board and the French thread building slowly, episodes three and four have to keep several London subplots warm, and a couple of them are treading water. Season 3 had no such gap.

It is worth noting where the series stood by this point: Will Smith won the 2024 Primetime Emmy for Outstanding Writing for a Drama Series for his work on the previous season, and the show had become a fixture in the Drama Series conversation. Season 4 is the one that spends that credibility on something genuinely uncomfortable rather than something crowd-pleasing.

👨‍👧 The dad perspective

Six episodes, around 45 minutes each, and this is not the season to put on for a light Tuesday. It is the one that will follow you upstairs.

The reason it hits a parent particularly hard is that it runs two generations of the same anxiety at once. There is a man watching his grandfather disappear in real time and being asked to make decisions on his behalf. And there is a man discovering that the shape of his own life was set by choices made about him before he could object. If you have ageing parents, or children you are quietly shaping without meaning to, both halves of that will find you.

Age guidance: 16 and up, higher than the previous seasons. The opening bombing is genuinely distressing, the depiction of dementia is unflinching, and the finale’s violence involves characters the audience has spent four years with. This is not a shared-sofa show for younger teens.

Rewatch value: moderate, and specific. This is the least comfortable season to return to, but the David Cartwright material is worth revisiting for Pryce alone. On an Apple TV 4K feeding a system with any kind of surround path, the opening episode’s sound work is a genuine showcase.

✅ Pros & cons

Need something lighter to follow it with? Prime Video’s 30-day free trial will cover a palate-cleanser evening — just check the current catalogue rather than trusting it to have anything specific.

Is Slow Horses Season 4 worth watching?

Yes, and it is the best-acted season of the show. We rate it 4.5 out of 5. Jonathan Pryce and Hugo Weaving are both exceptional, and the emotional stakes are the highest the series has attempted. Be warned that it is also the bleakest season.

Which book is Slow Horses Season 4 based on?

Spook Street, the fourth Slough House novel by Mick Herron, published in 2017. The adaptation keeps the book’s central premise of a retired spymaster whose failing memory makes him a liability.

Who does Hugo Weaving play in Slow Horses?

Hugo Weaving plays Frank Harkness, a former CIA operator running an off-books programme that raises and trains children as operatives. He is the closest thing the series has had to a conventional villain, and Weaving plays him with unsettling calm.

Does someone die in Slow Horses Season 4?

Yes. Without naming them, the finale takes a member of the Slough House team during an assault on the building, and the series carries the loss forward rather than resetting. It is the hardest death in the show so far.

How many episodes is Slow Horses Season 4?

Six episodes, released weekly on Apple TV from 4 September to 9 October 2024, each running roughly 45 minutes.