For five seasons, Slow Horses has kept viewers on the edge of their seats with its blend of gritty espionage and dark humor. Each installment has raised the stakes, introducing new threats and heartbreaking losses for the dysfunctional team at Slough House. Now, the newly released Season 6 trailer delivers a jaw-dropping revelation that promises to shake the series to its core: the return of a character many thought was gone for good.
A Familiar Face Returns
In the first season, River Cartwright (Jack Lowden) found an unlikely ally in Sidonie Baker, played by Olivia Cooke. Sid was a fellow Slough House agent who helped River investigate a kidnapping plot, but her true mission was to secretly monitor him for MI5's ruthless boss, Diana Taverner (Kristin Scott Thomas). Their partnership took a tragic turn when Sid was shot in the head by a masked assailant, leaving her in a coma. River visited her in the hospital, wracked with guilt, and later Taverner informed Jackson Lamb (Gary Oldman) that Sid had died. Yet, her body was never shown, and all records of her existence were wiped from MI5's systems—a detail that fueled fan theories for years.
Now, the Season 6 trailer confirms those suspicions. As Lamb quips that every spy has "faked their death at one time or another," the footage cuts to River climbing a stairway, only to find Sid standing at the top, very much alive. The shock on River's face mirrors what audiences will feel, especially since the series has largely moved on without her. For those who've read Mick Herron's source novels, Sid's return is no surprise, but for the rest of us, it's a game-changer.
What's Next for Slough House?
Season 6 is set to adapt two of Herron's novels, packing even more drama into just six episodes. The trailer reveals that the agents of Slough House—both current and former—are being hunted and executed, putting everyone in peril. Louisa (Rosalind Eleazar) is back, and River's father, Frank Harkness (Hugo Weaving), also makes an appearance, promising additional chaos. With such a massive reveal already in the trailer, it's clear the season will be packed with twists and turns that could rival anything the show has done before.
Fans have plenty of time to rewatch previous seasons and refresh their memories about Sid's role in the Slough House saga. Her return is a testament to the show's willingness to take risks, and it's an exciting prospect to see Cooke—who's currently captivating audiences on House of the Dragon—back in this world. As we await the premiere, one thing is certain: Slow Horses Season 6 is shaping up to be its most explosive yet.
All five seasons of Slow Horses are available to stream on Apple TV+ in the U.S., with Season 6 premiering September 16. For more on the show's upcoming slate, check out Apple TV's fall 2026 lineup. And if you're in the mood for more spy thrills, Lioness Season 3 is currently dominating Paramount+.
