I have been watching Slow Horses from the beginning, back when it still felt as though hardly anyone was paying attention to that strange Apple TV series about a group of failed MI5 agents working out of a decaying office under the command of the most unpleasant — and irresistible — man on television. Six seasons later, everyone has finally discovered what was right in front of us since 2022: Slow Horses is simply one of the best television shows.

Smart, funny, cruel, tense, brilliantly written and led by a Sir Gary Oldman who has turned Jackson Lamb into one of the great characters of contemporary television, the series has grown without losing almost anything that made it special. And now the Season 6 trailer has managed to do something I genuinely did not expect: besides making me count the days until we return to that filthy office, it has brought Olivia Cooke back.

Yes. Sid Baker.

Alive.

Sorry, I couldn’t help it. Back to Slow Horses: that simple “Hello, River” at the end of the trailer is a small bombshell for anyone who has been following the series since Season 1.

Sidonie Baker was one of the first characters we learned to like in that group. Far too competent to be at Slough House, intelligent, quick and clearly hiding something, she had with River Cartwright, played by Jack Lowden, a mixture of competition, complicity, attraction and suspicion that seemed destined to grow.

Until she was shot in the head in Episode 2. And disappeared.

Slow Horses moved on. So did we. Other agents arrived, other conspiracies, other deaths and countless humiliations handed out by Jackson Lamb. Olivia Cooke went elsewhere too. More specifically, she went to Westeros, where she became Alicent Hightower in House of the Dragon.

And now, ta-da: Sid is back.

The best part is that this is not a case of resurrecting a beloved character simply because Olivia Cooke became even more famous after leaving the series. The possibility was always there. Season 6 adapts two of Mick Herron’s novels, Joe Country and Slough House, and it is precisely in the latter that Sid’s story becomes important again.

For viewers who have only followed the television adaptation, Season 1 deliberately left everything in a grey area. Sid was taken to hospital in critical condition after being shot. Later, when River tried to find out what had happened to her, her records had vanished from the system. There was no body, no funeral, no confirmation that definitively closed the story.

She simply disappeared. In the books, we know that Sid survived, went through a long recovery, and returns carrying the physical and cognitive consequences of the trauma. So her reunion with River is not simply the return of someone he believed to be dead. It is the return of someone who is probably no longer exactly the woman he once knew. And that is very Slow Horses.

This has never been a series about flawless spies. It is about damaged, flawed, frequently humiliated people trying to keep working despite the weight of what they have done, lost or failed to prevent. River has spent several seasons accumulating professional and personal ghosts. Now one of them simply reappears in front of him. Jack Lowden’s face in the trailer says almost everything.

Olivia Cooke never forgot Sid either. Speaking about her return, she explained that from the early years of the series she knew there was a possibility, based on the books, that the character could come back. Over time, the production continued checking her availability until it finally managed to fit Sid back into her schedule. Cooke filmed Season 6 in late 2024, for roughly five weeks, keeping the secret while continuing to work on other projects. And there is something especially fun about seeing her return after spending so many years as Alicent.

Cooke spent that period immersed in another of television’s great series, playing a queen, the mother of kings and one of the central figures in the Targaryen civil war. Now she returns to the dry humor, sarcasm and speed of Slow Horses, a register that suits her wonderfully.

There is also a delightful coincidence for anyone who follows both shows. Olivia Cooke was not the only actor to leave Slow Horses and end up in Westeros. Freddie Fox, who played the memorably unbearable James “Spider” Webb, resurfaced in the second season of House of the Dragon as Ser Gwayne Hightower, none other than Alicent’s brother. For a few years, then, two characters who started out together in Jackson Lamb’s universe ended up reunited as Hightower siblings in the middle of the Dance of the Dragons.

Now only Sid gets to make the journey back. Spider met a far more definitive fate in Slow Horses, dying in Season 3 during his confrontation with Sean Donovan and later being thrown onto the pavement outside the restaurant where Lamb was sitting. Sid, who had also seemed to become one of the show’s earliest casualties, had in fact simply disappeared. And her return comes at a moment when Season 6 itself seems ready to reopen old wounds.

The story puts the Slow Horses on the run and draws them into another deadly game connected to Diana Taverner. Frank Harkness, River’s father, is still around. And combining Joe Country and Slough House into only six episodes suggests an especially intense season. It is fascinating to think about the journey Slow Horses has taken.

When it premiered in 2022, it felt like one of those excellent series that a few people discovered and immediately began recommending to everyone else. Apple was releasing seasons at an almost absurd pace, and for a while there was a sense that the show was much better than the size of the conversation surrounding it. Slowly, that changed.

People discovered Jackson Lamb. They discovered Jack Lowden. They discovered that vicious British humor. They discovered that an espionage series did not need to turn every agent into a version of James Bond to be thrilling. And Slow Horses went from being a delicious little secret to one of the most celebrated television shows. For those of us who were there from the beginning, there is a particular pleasure in that. But there is something even better about looking at Sid Baker again.

While we kept following River, Lamb, Diana Taverner, Catherine Standish and the rest, assuming that chapter was over, Olivia Cooke crossed over into another of television’s biggest productions, became Alicent Hightower and spent years surrounded by dragons, crowns, impossible children and civil war.

And then she returns to Slow Horses with two words: “Hello, River.” That’s it.

I’m already sold. September 16 cannot come fast enough.