‘Slow Horses’ Season 6 Trailer: Olivia Cooke Returns as Sid Baker

Meredith Jacobs August 18, 2026, 9:00 am

Olivia Cooke as Sid — 'Slow Horses' Season 6

Slow Horses continues to be a hit for Apple TV, with the adaptation of Mick Herron‘s Slough House book series coming back for (at least) two more seasons. And the sixth season, which premieres in September, of the espionage drama starring Gary Oldman will feature a major return: Olivia Cooke as Sid Baker.

The streamer announced that news on Tuesday, August 18, and released a trailer that ends with a look at her back and surprising River (Jack Lowden). After all, as Lamb (Oldman) puts it, “We’re spies. We’ve all faked our own death at one time or another.” Sid was shot in the head in the first season.

The video, which you can watch above, also offers a look at anyone who’s ever worked at Slough House being targeted. That means that Louisa (Rosalind Eleazar), who might have left and be in a new job, is being brought back into the fold.

“Do you know anything about why my Slow Horses keep turning up dead?” Lamb asks Diana (Kristin Scott Thomas) after turning up in her garden. She tries to brush it off as a coincidence, but he refuses to accept that.

“I cannot emphasize this enough: Stay dark. No phones,” he tells everyone. It’s no real surprise who then answers a call.

Jack Lowden as River, Gary Oldman as Lamb — 'Slow Horses' Season 6

Apple TV

Also back this season is Hugo Weaving as River’s dad, Frank Harkness. “Business” brings him back into his son’s orbit, he claims.

Read on for everything we know so far about Slow Horses Season 6, including the books on which it will be based, the cast, and more.

When does Slow Horses Season 6 premiere?

Once again, it will be a weekly drop. Slow Horses Season 6 will premiere on Wednesday, September 16, with one episode. The finale will begin streaming on October 21.

Apple TV announced that Slow Horses will be back for a sixth season in October 2024, less than a week after the fourth season’s finale began streaming.

“Audiences around the world have fallen in love with the Slow Horses, and I’m delighted that Gary Oldman will be leading this star-studded cast on another acerbic and action-packed adventure,” said Jay Hunt, creative director, Europe, Apple TV+, in a statement at the time.

The drama has gotten early renewals from the start, with, for example, the Season 5 pickup coming just days after the Season 3 finale began streaming.

'Slow Horses' Team Reveals Season 5 Scene You'll Never See (VIDEO)

On which book is Slow Horses Season 6 based?

Season 6 will actually cover two books, the sixth and seventh in Mick Herron’s series, Joe Country and Slough House.

The Joe Country synopsis reads as follows: “In Slough House, the London outpost for disgraced MI5 spies, memories are stirring, all of them bad. Catherine Standish is buying booze again, Louisa Guy is raking over the ashes of lost love, and new recruit Lech Wicinski, whose sins make him an outcast even among the slow horses, is determined to discover who destroyed his career, even if he tears his life apart in the process. Meanwhile, in Regent’s Park, Diana Taverner’s tenure as First Desk is running into difficulties. If she’s going to make the Service fit for purpose, she might have to make deals with a familiar old devil… And with winter taking its grip, Jackson Lamb would sooner be left brooding in peace, but even he can’t ignore the dried blood on his carpets. So when the man responsible for killing a slow horse breaks cover at last, Lamb sends the slow horses out to even the score.”

And the Slough House synopsis is: “At Slough House — MI5’s London depository for demoted spies — Brexit has taken a toll. The “slow horses” have been pushed further into the cold, Slough House has been erased from official records, and its members are dying in unusual circumstances, at an unusual clip. No wonder Jackson Lamb’s crew is feeling paranoid. But are they actually targets? With a new populist movement taking hold of London’s streets and the old order ensuring that everything’s for sale to the highest bidder, the world’s a dangerous place for those deemed surplus. Jackson Lamb and the slow horses are in a fight for their lives as they navigate dizzying layers of lies, power, and death.”

What is Slow Horses Season 6 about?

According to Apple TV, it “sees the Slow Horses on the run as Diana Taverner embroils them all in a fatally high-stakes game of retaliation and revenge.”

Who’s in the Slow Horses Season 6 cast?

The Season 6 cast consists of Gary Oldman (Jackson Lamb), Kristin Scott Thomas (Diana Taverner), Jack Lowden (River Cartwright), Saskia Reeves (Catherine Standish), Rosalind Eleazar (Louisa Guy), Christopher Chung as (Roddy Ho), Aimee-Ffion Edwards (Shirley Dander), Joanna Scanlan (Moira Tregorian), Samuel West (Peter Judd), Ruth Bradley (Emma Flyte), Tom Brooke (J.K. Coe), Jonathan Pryce (David Cartwright), and Hugo Weaving (Frank Harkness), with new addition Lenny Rush. Plus, Olivia Cooke is back as Sidonie “Sid” Baker.

It doesn’t look like James Callis will be back as Whelan, which was to be expected since he’s no longer First Desk. However, this isn’t the last we see of him in the books.