Apple TV has released the official trailer for Slow Horses Season 6, and Slough House may finally have found an enemy capable of doing something previously considered almost impossible.
Making Jackson Lamb genuinely worried.
Gary Oldman returns as the spectacularly unpleasant but exceptionally capable MI5 veteran, once again leading the collection of disgraced intelligence officers assigned to Britain’s least prestigious spy department.
This time, however, the Slow Horses are not simply investigating another threat.
They are the targets.
The trailer reveals that current and former members of Slough House are being systematically hunted, forcing Lamb to confront MI5 director Diana Taverner with an extremely uncomfortable question.
Why do his people keep turning up dead?
Kristin Scott Thomas returns as Taverner, and Apple’s official synopsis makes clear that she will play a central role in the escalating crisis.
Season 6 sees Diana embroil the Slow Horses in what the studio describes as a fatally high-stakes game of retaliation and revenge.
The threat quickly becomes serious enough for Lamb to order his team into hiding.
Anyone who has ever worked at Slough House may be a target.
Phones are supposed to stay switched off.
Agents are supposed to remain inside a safe house.
And everyone is expected to follow instructions.
Which, given the people involved, is already an optimistic plan.
Jack Lowden returns as River Cartwright, still carrying the familiar combination of genuine talent, terrible judgement and an almost supernatural ability to find himself at the centre of situations that would be much easier if he simply did what Lamb told him.
The trailer suggests that River will again be heavily involved in the action as the threat surrounding Slough House becomes increasingly personal.
Rosalind Eleazar also returns as Louisa Guy, while Saskia Reeves is back as Catherine Standish.
Christopher Chung, Aimee-Ffion Edwards, Tom Brooke, Joanna Scanlan, Samuel West, Ruth Bradley, Jonathan Pryce and Hugo Weaving are all among the returning ensemble.
Lenny Rush joins the new season as an additional member of the cast.
But the trailer saves its largest surprise for the final moments.
Olivia Cooke is back as Sid Baker.
Sid was one of the original Slough House agents introduced in Season 1 and appeared to have been killed after being shot in the head.
River was later told that she had died.
However, questions surrounding Sid never completely disappeared.
Her records were mysteriously removed from MI5 systems, leaving open the possibility that the official story surrounding her death was not entirely true.
Season 6 finally answers that question.
As Lamb casually observes that spies have occasionally been known to fake their own deaths, River looks towards the top of a staircase.
Sid is standing there.
Alive.
Her simple greeting to River instantly transforms one of the longest-running unresolved mysteries in the series into one of Season 6’s most important storylines.
Cooke has not appeared in Slow Horses since the first season, making her return particularly significant for viewers who have followed the series from the beginning.
The reunion also restores one of the show's original relationships.
Sid and River had an unusually complicated dynamic inside Slough House, combining professional rivalry, mutual respect and unresolved emotional tension.
Several years have now passed.
River believed Sid was dead.
Sid apparently had reasons for allowing him to continue believing exactly that.
There are going to be questions.
Probably quite a lot of them.
The wider plot, meanwhile, appears considerably more dangerous than a simple reunion.
The trailer shows assassins targeting Slough House personnel while Lamb attempts to identify who has placed his agents on what effectively amounts to a kill list.
Kyle Soller and MyAnna Buring appear among the figures connected to the new threat, suggesting that the team will face organised opponents rather than another accidental intelligence disaster.
Season 6 adapts material from two novels in Mick Herron’s acclaimed Slough House series: Joe Country and Slough House.
That combination allows the television adaptation to draw together several storylines involving River’s increasingly complicated family history, the enemies surrounding Slough House and the consequences of years spent operating inside the less respectable corners of British intelligence.
Hugo Weaving also returns as Frank Harkness, River’s father and one of the more dangerous figures introduced into the series.
The personal relationship between River, Harkness and the wider intelligence world gives the new season another layer beyond the immediate assassination plot.
Slow Horses has always worked because its threats are serious while its protagonists frequently are not.
Jackson Lamb may be brilliant, but he is also deliberately offensive, permanently dishevelled and seemingly determined to make every room smell worse after he leaves it.
His agents are capable spies whose careers were derailed by mistakes, personality flaws, bad luck or some combination of all three.
Yet when genuinely dangerous situations emerge, these supposedly failed agents repeatedly prove considerably harder to kill than their reputations suggest.
Season 6 appears ready to test that reputation directly.
Someone is hunting the Slow Horses.
Lamb does not know why.
Taverner appears to know considerably more than she is saying.
Old enemies are returning.
And one agent everyone believed was dead has suddenly walked back into River Cartwright’s life.
The new season again consists of six episodes.
Apple TV will premiere the first episode globally on September 16, 2026, followed by one new episode every Wednesday through the season finale on October 21.
The series has already been renewed for Season 7, which will adapt Mick Herron’s Bad Actors.
For now, however, Slough House has a more immediate problem.
Not everyone is expected to make it out alive.
Slow Horses Season 6 premieres September 16 on Apple TV.
Watch the official trailer below.
