“Slow Horses” opens with a man who has already lost. River Cartwright, an MI5 agent, botches a training exercise at a busy London station, and the failure costs him his career. He is not fired. He is sent to Slough House, a shabby building where the service parks its rejects — agents who failed badly enough to be punished but not badly enough to be let go. They are called slow horses, a play on the name of the place and on what their colleagues think of them.
That is the premise of the Apple TV+ series, adapted by Will Smith from the Slough House novels by Mick Herron. It premiered on 1 April 2022. The sixth series, based on the novels “Joe Country” and “Slough House,” premiered on 16 September 2026.
Trailer, via Apple TV.
Jackson Lamb Runs a Room Full of Rejects
The boss of Slough House is Jackson Lamb, played by Gary Oldman. Lamb is obnoxious, unkempt and brilliant, a man who anticipates that his staff will quit out of boredom or frustration. He is wrong. The slow horses keep getting pulled into cases that threaten Britain, and they keep working them.
The unit’s job is drudgery. Routine tasks, verbal abuse from Lamb, the daily reminder that they are not wanted. That is the design. The service expects them to endure it or leave. What the series does well is show why they stay: not loyalty to MI5, but the stubborn fact that they are still spies, and still want to be.
Oldman plays Lamb without vanity. He is not a mentor figure and not a secret hero. He is a man who has been in the game long enough to know what it costs, and who has decided the cost is worth paying. The performance is the anchor of the show, and it never turns sentimental.
Jack Lowden Leads a Deep Bench
Jack Lowden plays River Cartwright, the agent whose fall opens the series. River is the audience’s way in: he is talented, impatient and convinced he can earn his way back. Kristin Scott Thomas plays Diana Taverner, the MI5 executive who decides who gets sent to Slough House and who gets used by it.
The supporting cast is deep. Saskia Reeves plays Catherine Standish, Lamb’s assistant and the closest thing the unit has to a conscience. Christopher Chung plays Roddy Ho, a hacker with an ego that outpaces his social skills. Rosalind Eleazar plays Louisa Guy and Aimee-Ffion Edwards plays Shirley Dander, two agents whose competence is consistently underestimated by everyone outside the building.
Hugo Weaving joins as Frank Harkness, an antagonist who gives the later seasons a face to push against.
The casting was not an afterthought. Oldman was announced alongside the series order in October 2019. Olivia Cooke, Jonathan Pryce, Kristin Scott Thomas and Jack Lowden were added in December 2020.
One Novel Per Season, No Shortcuts
“Slow Horses” is a rare adaptation that treats its source material as a spine rather than a suggestion. Each season adapts a Mick Herron novel, and the show has kept pace with the books.
The first five series adapted the novels “Slow Horses,” “Dead Lions,” “Real Tigers,” “Spook Street” and “London Rules,” respectively. The sixth series, based on the novels “Joe Country” and “Slough House,” premiered on 16 September 2026. The series has been renewed for a seventh and eighth series, based on the novels “Bad Actors” and “Clown Town.”
That is an unusual amount of runway for a streaming series. Spy fiction lives and dies on plot, and Herron’s plots are the kind that reward patience. The show trusts that patience.
Why the Drudgery Matters
The central mechanic of “Slow Horses” is that the unit is a punishment, not a team. Slough House exists to make people quit. Every season, the slow horses get dragged into something that endangers Britain, and every season they do the work that the people above them will not.
The show never pretends this is glamorous. The building is ugly. The tasks are dull. Lamb’s abuse is constant, and the series does not soften it into banter. What makes the slow horses worth watching is that they are competent in a system that has decided they are not.
That is the show’s real subject: the gap between what an institution says about a person and what that person can actually do. It is a spy story, but it is also a workplace story, and the workplace is toxic by design.
What the Show Gets Right About Spy Work
Most spy television is about competence. “Slow Horses” is about the lack of it, and about what happens when the people who lack it are the only ones left. The slow horses are not superhuman. They make mistakes. The show does not spare them the consequences.
The tradecraft is grounded. The politics inside MI5 are petty and recognizable. Diana Taverner is not a villain; she is an executive protecting the institution, and Kristin Scott Thomas plays her as someone who has made peace with that. Frank Harkness is a villain, and Hugo Weaving plays him with the calm of a man who has been doing this longer than anyone around him.
The show also understands that the best spy stories are about loyalty between people who have no reason to be loyal. The slow horses are not friends. They are colleagues who have been thrown together and who keep choosing each other anyway. That is the emotional engine, and it works.
The Novels Behind Each Series
The show’s structure is unusually simple to track. Each series maps to one or two books, in publication order.
- Series 1 — “Slow Horses”
- Series 2 — “Dead Lions”
- Series 3 — “Real Tigers”
- Series 4 — “Spook Street”
- Series 5 — “London Rules”
- Series 6 — “Joe Country” and “Slough House”
- Series 7 — “Bad Actors” (renewed)
- Series 8 — “Clown Town” (renewed)
That list is the whole plan. Nothing has been invented for television that Herron did not already write.
The Case for Starting It
“Slow Horses” earns its reputation on craft rather than on premise. Gary Oldman plays Jackson Lamb, a character who could have been a caricature, and the show’s moral center. Jack Lowden, Kristin Scott Thomas, Saskia Reeves, Christopher Chung, Rosalind Eleazar and Aimee-Ffion Edwards all do work that would carry a lesser show on their own.
The series has a published critical score of 9.8 out of 10 on the Rotten Tomatoes Tomatometer. That is the number the show has to live up to, and it has kept pace with it across six series.
The one thing worth watching going forward is the ending. The show has been renewed through an eighth series, and it has the novels to get there. Spy series often collapse in their final act because they run out of story or lose their nerve. “Slow Horses” has neither problem.
That is rare. That is the reason to start this show if you have not, and the reason to finish it if you have.
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