The news was revealed as the streamer released a first-look trailer for the new season, which ends with River being shocked by Sid's return.
To recap, Cooke appeared as Sid in the first season of Slow Horses, but partway through she was shot in the head. She appeared at one point to have died, but Roddy Ho told River that he had uncovered evidence suggesting she could still be alive and out there somewhere.
Now, five seasons later, it has seemingly been confirmed that was the case. You can watch the full trailer for season 6, which is based on Herron's books Joe Country and Slough House, right here now.
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As well as confirming Cooke's return, the trailer also shows us plenty of action and reveals more story details, as Jackson Lamb confronts Diana Taverner about why the Slow Horses have started turning up dead – seemingly at the hands of two assailants played by MyAnna Buring and Kyle Soller.
We also get a glimpse of Paul Higgins, apparently back as his season 1 character, Struan Loy.
Also in the cast for season 6 are Gary Oldman, Jack Lowden, Dame Kristin Scott Thomas, Saskia Reeves, Christopher Chung, Aimee-Ffion Edwards, Rosalind Eleazar, Joanna Scanlan, Samuel West, Ruth Bradley, Tom Brooke, Sir Jonathan Pryce, Hugo Weaving and Lenny Rush.
The new season will premiere globally on Wednesday 16 September, with episodes being released weekly from that point until 21 October 2026.
The synopsis says: "Season 6 sees the Slow Horses on the run as Diana Taverner embroils them all in a fatally high-stakes game of retaliation and revenge."
Cooke's return and the choice to adapt two books rather than one aren't the only noteworthy factors at play in this season, as it is also the first without showrunner Will Smith. His departure was announced last year, with Gaby Chiappe acting as showrunner this time around.
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