Before anything else, the warning needs to be loud and clear: this article contains full spoilers for Joe Country and Slough House, the two Mick Herron novels that serve as the basis for the sixth season of Slow Horses. But there is an equally important caveat. The series does not appear to be simply putting the two books on screen, one after the other. Apple has confirmed that the six episodes adapt books six and seven of the saga, but the trailer and everything released so far suggest reorganized storylines, repositioned characters, and a plot that seems to bring the threat against the Slow Horses forward much earlier. So what follows is what happens in the books — and, therefore, a map of the dangers that may appear in the new season.

And perhaps that is better, because if the series reproduced everything Mick Herron wrote in those two novels exactly as it happened, we would already know far too much. There are major deaths, a return we have been waiting for since season one, and one of the cruelest cliffhangers ever reserved for River Cartwright. The trailer, however, already makes it clear that season six has its own architecture: officially, Diana Taverner draws the Slow Horses into a deadly game of retaliation and revenge, while Sid Baker, once again played by Olivia Cooke, is very definitely back.

It All Begins With David Cartwright’s Funeral

In Joe Country, River loses the man who raised him. David Cartwright dies in a care home after a long decline caused by dementia, bringing to an end a relationship that was always much more than grandfather and grandson. David was a father, a mentor and, above all, the person who built River’s almost romantic idea of what it meant to belong to the Service.

The funeral, naturally, offers no peace at all. River spots Frank Harkness watching the ceremony and runs after him. Frank escapes, but the image already says almost everything about River’s arc: at the exact moment he buries the man who represented his idealized version of espionage, the biological father whose existence shattered everything River thought he knew about his own origins reappears.

In the book, Harkness is tied to another plot that begins when Clare, Min Harper’s widow, contacts Louisa Guy because her teenage son, Lucas, has disappeared. Louisa still carries a deep connection to Min’s family and goes looking for the boy, eventually reaching Wales. Little by little, she discovers that Lucas had been working as a waiter at a private event attended by extremely rich and powerful people and witnessed a grotesque situation involving a young woman and a crossbow. He tries to turn what he saw into blackmail and ends up in the sights of people willing to pay to make the whole thing disappear. Frank Harkness and his mercenaries become part of that story.

This is exactly where I would be more careful about translating the book directly into the television version. The trailer is not presenting this plot as the centre of the season. The adaptation appears to have redistributed a great deal of Joe Country and blended its action with the manhunt from Slough House. So, for now, Lucas, Wales, and the exact circumstances of Harkness’s mission belong to the book spoilers, not to anything confirmed for the series.

Emma Flyte Dies in the Book

And this is where the losses begin.

In Joe Country, Diana Taverner is already running MI5 and attempts to demote Emma Flyte, head of the Dogs. Emma refuses to accept the new position and leaves the Service. Once free of the Park, she ends up helping Louisa find Lucas and goes to Wales just as the situation becomes dangerous.

Emma and Louisa even exchange coats, an apparently insignificant detail that ends up having fatal consequences. One of Harkness’s men finds Emma, tries to get information about Lucas, and shoots her. Emma Flyte dies.

It is one of the deaths I would be least willing to assume will happen exactly the same way on television. Ruth Bradley quickly became an excellent presence in the series and is confirmed for season six. Since the writers are already changing the structure of both novels, Emma may die in the same way; she may die under completely different circumstances, or she may simply survive.

And that is precisely the suspense readers of the books will have this time as well: knowing a character’s original fate no longer necessarily means knowing what will happen to them in the series.

Coe Does Not Survive Joe Country Either

J.K. Coe is another casualty in the book. When Louisa disappears, River, Shirley and Coe go after her and end up in a confrontation with Harkness’s mercenaries. Coe fights Cyril Dupont in a particularly brutal struggle. He manages to cut Dupont’s throat, but is stabbed as well and dies.

So Joe Country alone already provides enough material for a devastating season: David Cartwright dies, Emma Flyte is murdered, and Coe is killed too.

Then Slough House begins. And things get worse.

When the Slow Horses Become the Targets

At the end of Joe Country, Roddy Ho realizes that something very strange has happened: every record of anyone connected to Slough House, past or present, has disappeared from MI5 systems. It is as if they never existed at all.

In Slough House, we discover just how dangerous that is. Diana Taverner has secretly authorized the assassination of a Russian agent in an operation that could not go through official channels. The story also involves Peter Judd, private financing, and a chain of decisions that ends with a list of Slough House names falling into Russian hands.

They interpret the list completely wrongly: they conclude that Slough House must be some kind of secret British assassination unit. And they start killing the people on it.

It is a brilliant idea for Slow Horses because it turns the show’s original joke upside down. Slough House has always been the place MI5 sends agents it would rather forget. Suddenly, being forgotten by the Service is no longer merely humiliating. It is deadly.

Former Slow Horses begin turning up dead, and Lamb realizes someone is hunting his agents. From that point on, the man who spends his entire life insulting, humiliating, and pretending to despise his team reacts exactly as we expect him to: messing with his joes means messing with Jackson Lamb.

Sid Is Alive

It is also in Slough House that the return the series is no longer trying to hide takes place: Sid Baker survived. Olivia Cooke is officially returning for season six, and the promotional material itself shows Sid back with River.

In the books, after being shot in the head, Sid is treated, and her existence is erased by the Service. She starts living under another identity, far from everything, still dealing with the consequences of the injury. When she realizes two suspicious people are after her, she seeks out River because she still remembers him as someone she can trust.

There is something genuinely moving about that. River spent years not even knowing whether Sid was alive. When she finally returns, it happens just as everything he once believed was permanent begins to disappear. And the reunion does not last long before the two of them have to fight for their lives.

Sid is kidnapped by two killers, Jim and Jane, but manages to kill Jim with an old stiletto that belonged to David Cartwright. Jane continues hunting her and tries to drown her. River arrives, and he and Sid manage to defeat her.

It looks like they have finally escaped. Of course they haven’t.

River’s Fate Is the Biggest Cliffhanger

At the end of Slough House, River and Sid are at David’s old house. At some point, River uses the front door. Later, Sid realizes he has disappeared. She finds him collapsed on the kitchen floor, the victim of a nerve agent placed on the door handle. She calls for help and tries to keep him conscious as the book ends without telling us whether River Cartwright is alive or dead.

He is alive. The later novels confirm that River survives the poisoning, although the consequences are serious and his recovery takes time. But anyone who finished Slough House when it first came out did not have that reassurance. Mick Herron simply closed the book with River apparently dying in Sid’s arms. If the television series decides to use the same cliffhanger in episode six, it would be a particularly cruel way to end the season.

And there is a delicious irony now that Jack Lowden has again been mentioned in the conversation around James Bond. In Ian Fleming’s From Russia with Love, Bond also ends the novel poisoned, his fate left unresolved. River Cartwright, the ultimate anti-Bond, could end a season in exactly the same way.

So Who Will Actually Die?

That is the question that became much more interesting after the trailer.

The books offer several ready-made victims: David Cartwright, Emma Flyte, J.K. Coe and former members of Slough House. They also offer River very nearly dying at the end. But season six is compressing two large novels into just six episodes and, from everything we have seen so far, it is not preserving their structure intact. So knowing the books does not necessarily mean knowing who will die. And I think that is excellent.

Because Slow Horses works best when we feel that no one is completely protected. We know the source material, we recognize the pieces on the board, and we know bodies are coming. What we no longer know is who will take whose place when the series decides to move those pieces around.

And River may be the character with the most to lose. He begins this story burying David, the man who gave him an idea of heroism and of the Service. He comes face-to-face again with Frank, the father who represents the most brutal version of that same world. Sid returns from the dead. People around him disappear. And in the books, River is the one who ends up literally on the floor, suspended between life and death.

For years, he has tried to escape Slough House and return to Regent’s Park. Perhaps this is finally the season in which he discovers something we realized a long time ago: Slough House has become his family. David taught River to idealize the Service. Frank showed him how monstrous that world can become. Diana taught him that the institution will always place its own survival above the survival of its agents.

Perhaps Jackson Lamb, of all people, is the one who will finally teach him what it actually means to be a good spy. And now, even knowing everything that happens in the books, I genuinely do not know who will still be alive when we reach episode six.

Which, for Slow Horses, is exactly how it should be.