Sid Baker’s return understandably stole most of the attention when the trailer for Slow Horses Season 6 arrived. Olivia Cooke, or rather, Sid Baker, appearing in front of River Cartwright five seasons after being shot in the head, absolutely deserved a story of its own. But once the surprise settles, it is worth watching the trailer again, because there is a much bigger story being built around it: this time, Slough House is not stumbling into a conspiracy. Slough House is being hunted.

Apple had already confirmed that Season 6, premiering on September 16, will combine Joe Country and Slough House, the sixth and seventh novels in Mick Herron’s series, into just six episodes. That is an important change. Until now, the adaptation had been working with roughly one novel per season; this time, two separate stories will be merged into one. And the trailer begins to show us how.

The most important information comes from Jackson Lamb himself. The father of a young boy stole a file containing the names of the Slow Horses, then died, and now, according to Lamb, his “Joes” are being hunted. At the same time, Molly Doran discovers that Slough House has completely disappeared from MI5’s systems. Roddy Ho puts it even more simply: they no longer exist.

It is a perfect idea for this series. From the beginning, Slough House has been where the Service sends agents it would rather forget. They still belong to MI5, but they live in a kind of professional death, doing meaningless work under Lamb until they resign or simply accept that their careers are over. Season 6 seems to take that condition to its logical extreme: someone is not only trying to kill those agents, but also removing any evidence that they ever existed.

That is one of the major threads in Slough House. In the novel, the group realizes that their records have vanished from the system while people connected to the unit begin to be watched and eliminated. What first looks like paranoia slowly forms a much more frightening pattern: there is an operation underway against them, and the bureaucratic erasure is part of the same threat.

The difference is that the series reaches that story at a very different point from the books. Diana Taverner is now First Desk, after spending five seasons trying to get to the top. Apple’s official synopsis says that she will drag the Slow Horses into a deadly game of “retaliation and revenge”, so the question is not simply who is hunting the group, but which of Diana’s operations opened the door to all of this.

And this is where Peter Judd becomes important again. Since Season 1, he has been one of the series’ most consistent links between politics, intelligence, money and personal ambition. Judd comes and goes whenever MI5 becomes entangled in things that cannot be done cleanly or officially, and his connection to clandestine financial structures once again became important in Season 5. In the books, his relationship with Diana becomes even more dangerous precisely when she reaches the top and needs to operate outside the formal limits of the Service.

It is a thread Slow Horses has been weaving for years. Diana believes she can manipulate politicians, agents, and covert operations as long as she controls enough information. Judd, meanwhile, almost always sees chaos as an opportunity. Separately, they are dangerous enough. Together, they represent exactly the kind of mixture of political ambition and clandestine intelligence that usually ends with Jackson Lamb having to clean up the mess.

But Season 6 does not come only from Slough House. The other half is Joe Country, and that is where Frank Harkness, River’s father, played by Hugo Weaving, comes back into the picture. In the novel, Louisa Guy becomes involved in the search for a missing teenager while River crosses paths with Harkness again, and the two stories eventually converge. On television, some of that material has already been moved forward: Harkness appeared in Season 4, and the series has substantially reorganized the Cartwright family story. So we should not expect a literal adaptation.

The trailer suggests exactly that kind of fusion. The threat from Slough House — agents being hunted and the unit erased from the system — appears to provide the backbone of the season, while Joe Country contributes characters, conflicts and especially Harkness to feed into that pursuit. Louisa is also pulled back into the action despite having tried to move away from that world, which fits perfectly with a season in which even those who have left Slough House still seem trapped by Lamb’s list.

That may be the biggest narrative change of Season 6. Instead of adapting two novels one after another, the series seems to be taking them apart and rebuilding them as a single conspiracy. And that makes sense because Slow Horses has already moved so many pieces of Herron’s chronology that following the books rigidly would be almost impossible at this point. Mick Herron remains the foundation, but the adaptation has earned enough freedom to reorganize his universe.

There is also an important change behind the scenes. This will be the first season without Will Smith, who led the adaptation across the previous five seasons and was crucial in defining that very particular balance between espionage, violence, melancholy, and vicious British humor. Gaby Chiappe takes over the writing for Season 6, while Adam Randall, who directed Season 4, returns to direct the episodes. Judging by the trailer, at least, the tone still feels familiar: more action, chases and explosions, but Lamb, Roddy, River and Diana remain completely at home in the world we know.

And then, of course, there is Sid Baker. Her return belongs to the Slough House storyline and takes on a different meaning in this context: she reappears precisely when agents connected to the unit are being hunted, and their traces are beginning to disappear. But that is only one piece of a season that seems to be aiming at something much larger than the return of any single character.

The Season 6 trailer suggests a very interesting reversal. For five years, the Slow Horses survived because people underestimated them. They were the agents nobody wanted, forgotten in a building Regent’s Park preferred to pretend did not exist. Now someone seems to have realized that they exist very much indeed — and has decided to do something about it.

The irony is wonderful. For years, MI5 tried to bury Slough House. Now someone wants to finish the job. And to do that, they will have to get past Jackson Lamb.