'Slow Horses' Season 6 Officially Says Goodbye to a Beloved Character

Published Sep 17, 2026, 6:58 PM EDT

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Editor's note: The below contains spoilers for the Slow Horses Season 6 premiere.

There have been a lot of major losses on Apple TV's Slow Horses in its first five seasons. Season 1 saw Sid Baker (Olivia Cooke) exit the series after being shot in the head, while Season 2 broke our hearts when Min Harper (Dustin Demri-Burns) was murdered by Russian operatives. Perhaps one of the hardest deaths to witness was Marcus Longridge's (Kadiff Kirwan), who died in a wild shootout inside Slough House in Season 4. Season 6, which premiered this week, opens with another loss for the Slow Horses, as Struan Loy (Paul Higgins), who we last saw in Season 1, is brutally murdered by two mysterious assassins who seem to be targeting former Slough House agents. As shocking as Struan's murder is, it isn't even the most tragic death in the Season 6 premiere.

'Slow Horses' Season 6 Features Several Heartwrenching Deaths

In the aftermath of Struan's death at the hands of two assassins, River Cartwright (Jack Lowden) is revealed to be in a meeting with the now-head of MI5, Diana Taverner (Kristin Scott Thomas). River isn't there to receive a promotion after being relegated to Slough House in Season 1; he's actually there to plan a funeral for his grandfather, David Cartwright (Jonathan Pryce). Yes, in between the action of Season 5 and the Season 6 premiere, River's beloved father figure has passed away. It was a peaceful death in his assisted living facility, but the loss of such a significant character is still immense. A flashback scene later in the episode depicts David's final moments with River by his side. David confesses that he always tried his best to provide a happy home for River before confirming that he will miss River when he is gone. It is a heartbreaking scene because you can feel River's grief, as well as David's sadness at leaving his grandson behind.

As tragic as David's death is, there are plenty more salt-in-the-wound moments for River. He's unable to get his mother to call him back, and, of course, she doesn't attend David's funeral. When River delivers a thoughtful eulogy, he can't say what he truly wants to about his grandfather. Diana has given him specific instructions to obscure David's true occupation, so River has to pretend that his renowned spy grandfather actually worked for the Department of Transportation, making sure potholes were filled. His speech gives the always insensitive Jackson Lamb (Gary Oldman) the giggles, which makes the scene even more painfully awkward. All of this would be bad enough, except that things get worse when River's father, the villainous Frank Harkness (Hugo Weaving), shows up. The two men exchange words outside the church, but instead of a loving expression of support, Frank ends up punching River in the throat before driving off, with River attempting to cling to the hood of his car and ultimately falling onto the pavement.

River Cartwright Will Face a Difficult Path in 'Slow Horses' Season 6

David's death is certain to have a huge impact on River as Slow Horses moves forward. River's grandfather didn't just serve as a surrogate parent who never abandoned him; he also provided helpful guidance about working at MI5 given his illustrious career and once-brilliant mind for spycraft. In light of his passing, the already rudderless River is likely to face a serious identity crisis. Lamb isn't typically a great mentor, which means River will also feel alone in both his personal and professional life.

River likely won't have time to truly deal with his grief, either. Instead of taking any time off work for bereavement reasons, he's already helping Lamb investigate Struan's death. It's likely he'll get sucked back into spy work, especially now that he'll need to help the other Slow Horses figure out who is targeting and murdering their former comrades. With no true family left, River will throw himself into solving this case, but the impact on his psyche from not giving himself time to process his feelings will probably be disastrous. Two things are clear: River isn't getting out of this season unscathed, and we'll miss David popping up to guide him.

Slow Horses Season 6 is available to stream on Apple TV in the U.S. New episodes drop every Wednesday.

Slow Horses