Slow Horses season 4: All we know from plot details to new cast members and release date speculation

We're counting down the days until Slow Horses returns with its hotly-anticipated fourth season. Apple TV+'s hit espionage drama, based on Mick Herron's Slough House novel series, follows a group of misfit intelligence agents who have been banished to the dumping ground department of MI5 after making career-ending mistakes.

Season four was announced back in June 2022, when Apple TV+ green-lit two further seasons. But just when will the new episodes arrive and what can fans expect? Keep reading for all we know…

Season four release date speculation

While Apple TV+ has yet to announce an official release date, we do know that filming for season four has already wrapped up.

Speaking with HELLO! at the BAFTA Television Awards Nominees' Party in April, Christopher Chung – aka Roddy – revealed that the cast were in the middle of filming season five.

"We were filming [series four] right up until the strike happened. Then we picked back up at the start of this year before we went straight into season five," he explained.

Back in December last year, showrunner Will Smith said that the new season would be on our screens before the end of 2024. This means we can expect the new series to premiere on Apple TV+ in December at the latest.

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Slow Horses season four cast

Slow Horses just wouldn't be the same without Gary Oldman's Jackson Lamb, who in season four will be joined by Kristin Scott Thomas as Diana Taverner, Saskia Reeves as Catherine Standish, Jack Lowden as River Cartwright, Rosalind Eleazar as Louisa Guy, Christopher Chung as Roddy Ho, Aimee-Ffion Edwards as Shirley Dander, Kadiff Kirwan as Marcus Longridge and Jonathan Pryce as David Cartwright.

We also expect to see Samuel West reprise his role as Conservative MP Peter Judd, alongside Sophie Okonedo as disgraced ex-head of MI5 Ingrid Tearney.

Some fresh faces have also been added to the cast of season four, including BAFTA winner Joanna Scanlan (The Larkins), Hugo Weaving (The Lord of the Rings), Ruth Bradley (Guilt), Tom Brooke (Empire of Light) and James Callis (Battlestar Galactica).

What will happen in Slow Horses season four?

The new season will open with "a bombing that detonates personal secrets, rocking the already unstable foundations of Slough House".

Teasing the show during an exclusive chat with HELLO!, Christopher Chung revealed that the new episodes are "bigger, better and more exciting". "There's a lot of espionage in it and there's a lot more fart from Gary [Oldman]!" he joked, adding: "Everyone is in for a thrilling ride."

Fans can also expect a "great storyline" for Roddy, who "gets a girlfriend" in the new episodes, according to Christopher.

While Apple TV+ has yet to share any other details, we do know that the new season will be based on Mick Herron's novel Spook Street.

The blurb for the book reads: "Twenty years retired from the Intelligence Service, David Cartwright still knows where all the bones are buried. But when he forgets that secrets are supposed to stay hidden, there's suddenly a target on his back.

"The 'Old Bastard' raised his grandson to be a hero, not a slow horse. Now, far from joining the myths and legends of Spook Street, River Cartwright is part of Jackson Lamb's team of pen-pushing no-hopers at Slough House. Which doesn't mean he won't ditch everything and go rogue when his grandfather comes under threat.

"Lamb worked with Cartwright back in the day, and knows better than most that this is no innocent old man. So when a panic button raises the alarm at Intelligence Service HQ, it's Lamb who's called on to identify the body. And it's Lamb who'll do whatever's necessary to protect an agent in peril."