Slow Horses gradually gained traction, even if those who’ve followed the series since season one already knew how good it was. The production has plenty of books to keep the series going for years, and cleverly films two seasons at a time. That means YES, there’s already a new one ready to go, and it’s hitting Apple TV+ in September.

Throughout the first four seasons, we’ve seen many character changes — and lost a few along the way. The big turning point for River Cartwright (Jack Lowden) wasn’t landing in Slough House, but the discovery that his father, Frank Harkness (Hugo Weaving), was a deranged terrorist monster. A truth that will continue to haunt him.

Another theme to be explored is the advanced dementia of his grandfather, David Cartwright (Jonathan Pryce). Many Cold War secrets and morally murky decisions are still buried, and so far, only Jackson Lamb (Sir Gary Oldman) seems to grasp the magnitude. Meanwhile, the world keeps spinning — with terrorists and politicians eager to tear it apart.

If you don’t mind spoilers, let’s dig into what’s coming, since this season will adapt London Rules, the fifth book in Mick Herron’s Slough House series. It’s a thriller mixing terrorist attacks, political intrigue, and that classic blend of black humor and melancholy that defines the “slow horses.”

London Rules — All the secrets of a dirty game

The slow horses are still MI5’s misfits — agents sidelined to pointless tasks after field failures. Jackson Lamb continues to lead this dysfunctional crew with cynicism, flatulence, and a razor-sharp killer instinct. Though scorned by everyone, they’re the only ones who smell the rot where it truly festers.

The book opens with an attack in Derbyshire: a truck driver plows through a rural market, killing multiple people. Soon after, a popular Muslim imam is gunned down. The escalating violence hints at a pattern of coordinated terrorist attacks, prompting both MI5 and the public to panic.

But as always in Mick Herron’s world, nothing is what it seems.

Roddy Ho and the mysterious woman

Amid the chaos, IT agent Roddy Ho shows up with a “perfect girlfriend,” Leila — glamorous and way out of his league. The other slow horses immediately suspect something. And rightly so: Leila is connected to the armed terrorist group Sons of Albion, who plan to use Roddy as bait to breach MI5’s security systems.

But it’s not just a coincidence — Roddy was handpicked precisely because he’s seen as the weakest link, someone consistently underestimated. That becomes crucial to the enemy’s plan.

Meanwhile, the British government — especially First Desk Claude Whelan — faces mounting pressure from attacks with apparent political, racial, and religious motivations. It’s a powder keg. The true mastermind, however, is Peter Judd, a far-right politician seen in previous books. Judd is maneuvering to exploit the chaos as a path to power, with outside agents feeding the public’s fear. The goal is destabilization, panic, and an authoritarian backlash.

What do the slow horses do?

Louisa Guy (Rosalind Eleazar) grows suspicious of Leila and begins investigating her independently, clashing with MI5’s official plans, which are more concerned with self-preservation than saving lives.

Meanwhile, Jackson Lamb realizes the attacks are smoke and mirrors for a deeper coup: to destroy MI5’s credibility and install a more populist, authoritarian regime.

Lamb plays a double game — letting the terrorists think they’re winning while planting false leads and preparing the counterattack. As always, he uses the most unlikely pawns on the board.

In the book’s climax, there’s a direct attack on MI5: an attempted assassination of Claude Whelan, who narrowly survives. Acting outside official command, the slow horses dismantle part of the conspiracy, expose Leila’s betrayal, and stop the Sons of Albion from using Roddy to do more damage.

As always, Jackson Lamb manipulates the board to his advantage — even using blackmail to keep his team alive and preserve the unstable status quo that protects him.

Roddy Ho, surprisingly, is the centerpiece of the plot. Though still insufferable, his vulnerability reveals that even the most useless can become useful — or dangerous. Louisa Guy, still grieving Min Harper, shows more autonomy and focus.

Shirley Dander remains impulsive and aggressive but shines in action scenes. Jackson Lamb becomes ever more complex: beneath the grotesque behavior lies a ruthless strategic mind — and perhaps a twisted, but real, sense of justice. Meanwhile, Claude Whelan and Diana Taverner keep the “real” MI5 afloat amid sabotage, leaks, and internal warfare. Diana, ever the shadow player, stands out for her manipulations.

And River?

Still grappling with trauma and the fallout from previous seasons, River remains technically assigned to Slough House, but emotionally more isolated than ever — particularly under the shadow of his biological father, Frank Harkness. Though still offstage, Frank is increasingly referenced in ominous terms. He’s not just an ex-agent — he’s a dark legend in British espionage, tied to assassinations, illegal ops, and betrayal. His connection to River is an open wound, and everyone knows that if Frank returns, it will be to destroy something.

River’s involvement in London Rules is deeply personal: he’s trying to prevent more innocent bloodshed while distancing himself from his father’s legacy — a legacy that may live on in his own instincts.

Another key thread is the slow deterioration of David Cartwright. Once a brilliant agent known as “King of the Castle,” he’s now increasingly confused, paranoid, and vulnerable. He suspects MI5 is trying to eliminate him — and, to be fair, he’s not entirely wrong.

David still holds secrets that could incriminate powerful people — including, perhaps, Frank Harkness. Even in his senility, he’s a silent threat to Regent’s Park. The real danger is that David still knows how to manipulate the system — whether out of instinct, vengeance, or confusion.

River tries to protect him, but begins to realize he can no longer tell if his grandfather is truly losing his mind or pretending. Their fragile bond starts to fracture. The tragedy of the Cartwright line is that all the men seem destined to become irrelevant or dangerous — and River fears he might be next.

Thus, the season will explore timely themes like fake news and paranoia, as the attacks are staged to appear as Islamic terrorism but are in fact orchestrated internally — a direct critique of fear as a political weapon. It tackles manipulation of public opinion, with Peter Judd exploiting chaos to pursue the role of Prime Minister, while MI5 must decide whether to protect the nation or its own image. It also dives into the moral ambiguity of intelligence work, where, once again, Herron shows that MI5 lies, manipulates, and abandons its agents — and that maybe only the outcasts, the slow horses, retain a functioning moral compass.

London Rules maintains the acerbic, gripping, and surprisingly emotional tone of the series, offering a character study of damaged people and a fierce satire of British institutions. With dark, intelligent humor, it dares you to laugh — even when everything’s falling apart.

More than a terror plot, it’s an X-ray of a country on the verge of collapse — and a reminder that in London, the rules are made to be broken… or ignored entirely.