Slow Horses season 3 ending spoilers follow.
Mick Herron's gritty London-set Slough House novels have formed the basis of the darkly funny spy thriller series Slow Horses, starring Gary Oldman as Jackson Lamb, the dishevelled, sarcastic and often brilliant leader of a group of intelligence officers known as the 'slow horses'.
The team of MI5 rejects who have just stopped short of being sacked from the intelligence service (so far) are based at Slough House, where much of their work is dull and administrative – although they have a habit of stumbling across investigations that often threaten Britain itself.
The first three seasons introduced us to the inner workings of Slough House, as well as Lamb's eccentric ways, including a fondness for farting and a personal hygiene routine that involves a grubby towel and a bottle of washing-up liquid.
How anyone can stand to be in the same room with him is an ongoing puzzle we need solving, and more questions about Lamb, his office manager Standish and the team were raised in the season three finale that hopefully will be answered in the next season.
Read on for everything you need to know about Slow Horses season four.
Slow Horses season 4 potential release date: When can we expect Slow Horses season 4?
Filming for season four took place in the spring of 2023 and has been completed, but that doesn't mean we will get to see the Slow Horses tackle their next case any time soon.
While Slow Horses season one was released in April 2022, and season two followed the same year (December 2022), there was then a whole year's gap until the release of season three on November 29, 2023.
This means we'll probably have to wait until the end of 2024 before we get back to Slough House – and we can only imagine (with horror) just how messy Jackson Lamb's office (where he also sleeps) will be by then.
Showrunner Will Smith all but confirmed this during an interview with Decider back in December 2023, saying: "I can tell you that it won’t be later than this time next year. So you won’t have to wait longer than a year."
The good news is that Apple TV+ seems to be on board for the long haul. It announced in January 2024 that Slow Horses season five has been officially commissioned – which, if the past four seasons are anything to go by, means that season six is in the bag, too.
As Smith told The Watch podcast: "It's a pretty well-oiled machine but it's relentless because at any one point there are always three of them going on at once – one in the edit, one being prepped or shot and one being written.
"We shoot two back-to-back so it's like a year shoot, pretty much, by the time we've finished it – and during that time we'll edit and write another one."
That suggests there's no end in sight for the series, and Mick Herron has written eight books so far so there's plenty to work from.
Slow Horses season 4 cast: Who will be coming back?
A sneak peek at season four that pops up at the end of the season three finale reveals that many of our favourite Slow Horses will return for the next season.
Of course, Slough House wouldn't be the same without Gary Oldman as Jackson Lamb, and in season four he will be joined by returning cast members including:
• Jack Lowden as River Cartwright
• Kristin Scott Thomas as Diana Taverner
• Saskia Reeves as Catherine Standish
• Aimee-Ffion Edwards as Shirley Dander
• Kadiff Kirwan as Marcus Longridge
• Christopher Chung as Roddy Ho
• Rosalind Eleazar as Louisa Guy
• Jonathan Pryce as David Cartwright
While they're not in the trailer, it's also possible Samuel West may pop up as weaselly on-off Conservative MP Peter Judd, and Sophie Okonedo as Ingrid Tearney, the disgraced ex-head of MI5.
The preview also introduces us to Hugo Weaving as a potential bad guy (book fans will know about his connection to the Cartwright family...), along with James Callis as one of Taverner's associates at MI5 and Ruth Bradley as a new character we see being introduced to the 'delights' of Slough House by Jackson Lamb.
Slow Horses season 4 plot: What will happen?
The fourth season of Slow Horses is based on Mick Herron's novel Spook Street.
The official synopsis for the novel reveals that some of the action involves River's grandfather David, who began to show signs of memory loss and dementia in season three.
"Retired from the intelligence service, David Cartwright still knows where the skeletons are hidden. But when he forgets that secrets are supposed to stay buried, there's suddenly a target on his back… Jackson Lamb worked with David back in the day.
"He knows better than most that this is no innocent old man. So when River's panic button raises the alarm at Intelligence Service HQ, Lamb will do whatever he thinks necessary to protect an agent in peril."
Spook Street by Mick Herron (Slough House book 4)
There are also loose ends from the season-three finale to be tied up. At the end of the season, River leaked important documents that revealed some dodgy goings-on (including murder) at MI5 under Ingrid Tearney's leadership, and by the final scene, Taverner had stepped up to take disgraced Tearney's place as First Desk (the Director-General of MI5).
"Where it ends in season three, where Taverner thinks she's got what she wants, let's just say there's a recalibration of that at the start of the next series," showrunner Will Smith told Digital Spy in an exclusive interview.
"There's repercussions [from River's leak], but they're more left field than directly to do with what he leaks."
Related: Slow Horses boss shares why Gary Oldman looks worse every season
Before he leaked them to the press, River was shocked when he took the documents to his grandfather for advice and David threw them in the fire to destroy them (luckily River had copies in his car). Smith adds that their relationship changes following that visit in the new season.
"The main thing is the fracture that occurs when River is going against his grandfather for the first time, and that's a big thing for him, and seeing his grandfather in a new light and he is starting to make his own judgements and not just toe the line. The next series is sad with what happens between River and David," he said.
Catherine Standish, Slough House's reliable office administrator, also faces changes in the next season after Lamb shockingly told her that her former boss Charles Partner was a traitor who sold secrets to the Russians. She quit and walked away from Lamb and Slough House, but will be back in some capacity in season four.
It remains to be seen whether she discovers Lamb's secret (revealed to viewers in the first season) – that he killed Partner for his treachery on the orders of David Cartwright and made it look like a suicide.
"The only bit of the puzzle that she can put together is the fact that it was Lamb that did it and it's whether she intuitively knows that underneath it or it needs to be spelled out," Smith told Digital Spy.
"In the book, she's directly told at one point and we have the knock-on from that. We will probably get to that, yes."
As if that weren't enough drama to deal with, the season-four sneak peek reveals that a bomb goes off in London and MI5 and the Slow Horses have to investigate.
They're also in danger themselves – it's possible the sharp-tongued keeper of MI5's records, Molly (Naomi Wirthner), may not survive, while clips also show River being held at gunpoint and threatened by Hugo Weaving's mysterious character.
It's Jackson Lamb's closing line in the trailer that worries us most, however, as he says: "One of my team just died." Who could it be?
Slow Horses season 4 trailer: Any Slow Horses season 4 footage yet?
When you watch season three, at the end of the finale (episode six: 'Footprints') the lovely people at Apple TV+ deliver an extra treat – the first glimpse of season four. It's available to view right here:
Slow Horses is available on Apple TV+.
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Freelance film & TV writer, Digital Spy
Critic and writer Jo Berry has been writing about TV and movies since she began her career at Time Out aged 18. A regular on BBC Radio, Jo has written for titles including Empire, Maxim, Radio Times, OK!, The Guardian and Grazia, is the author of books including Chick Flicks and The Parents’ Guide to Kids’ Movies.
She is also the editor of website Movies4Kids. In her career, Jo has interviewed well-known names including Beyonce, Steven Spielberg, Tom Hanks, Kiefer Sutherland, Tom Cruise and all the Avengers, spent many an hour crushed in the press areas of award show red carpets. Jo is also a self-proclaimed expert on Outlander and Brassic, and completely agrees that Die Hard is a Christmas movie.
