With its emotional fourth season, ‘Slow Horses’ proves it’s the most exciting show on TV

There are few things that galvanize a down-on-his-luck agent wanting — no, needing — to prove himself more than the measured warning: “You should stay out of this.” So begins our introduction to River Cartwright (Jack Lowden) in the first season of the Apple TV Plus spy thriller “Slow Horses.”
River is an MI5 agent newly assigned to Slough House, the tranche where British intelligence agents get relegated if they make a mistake on the job. It’s a dumping ground of sorts, and River is adamant to prove himself a worthy agent and get back into good standings at Regent’s Park, the MI5 headquarters.
From the onset, the show is clear that the biggest obstacle in River’s way is his impulsivity and pride. He often talks back to superiors, discounts colleagues and disobeys orders from his boss Jackson Lamb (Gary Oldman). Yet, over four seasons, it is the curmudgeon Lamb who proves to be the guiding force for River, learning how to tame him but also how to use his frenzied instincts to uncover agency conspiracies and rescue innocents caught in the fray.
Lamb is a legend-turned-slob; he rarely wears his shoes inside his office so that whenever anyone from his team has the misfortune of calling upon him, they’re greeted by his big toe peeking out from a hole in his sock, propped comfortably on a stack of papers on his desk. He is a decorated spy whose flatulence dominates conversations and whose canary yellow car better resembles a taxi cab than a sleuth’s vehicle.
He’s a walking contradiction in that he’s all of these things — and more — and yet, fully competent at his job. He’s able to piece together evidence, make logical jumps and predict moves far faster than anyone on his team or MI5 altogether.
He’s sardonic and witty, and it is the funny exchanges between him and his team that provide the levity to a series whose agents are tasked with fighting white nationalist terrorism, police brutality and corruption. It’s an insult humour reminiscent of shows like “Succession,” “Veep” and “The Thick of It,” and by no coincidence: “Slow Horses” showrunner Will Smith was part of the writing team for the latter two shows.
Smith does an adept job at adapting Mick Herron’s “Slough House” novel series, rife with intrigue and suspense owing much to John le Carré. Since the show’s first season aired in 2022, he has presented Herron’s characters in their full-fledged, imperfect forms: the Slough House agents range from the self-flagellating to the self-obsessed, and each harbours the secret of the mistake that landed them in the reject pile of MI5.
When faced with an external threat — nationalist terrorism in the first season, Russian sleeper agents in the second — they mess up almost as often as they succeed: they’re the opposites of James Bond, Ethan Hunt, George Smiley or any other suave spy we’re used to encountering on our screens, cool, calm and collected. But they all share a drive to triumph, in spite of their failings, which allows them key moments of ingenuity and shrewdness.
And in a television ecosystem that has normalized three-to-four-year gaps between seasons, “Slow Horses” has reliably released a season every year; the fourth season aired its finale last week and marked the culmination of its best season to date.
There’s a certain structure to each season of “Slow Horses” that is predictable: an external threat presents itself, Regent’s Park attempts to handle it on their own, the Slow Horses inevitably get involved and play a large role in the resolution.
What makes Season 4 stand out is its artful grasp in balancing its characters’ personal and professional lives within this repeated pattern, something that the show has been somewhat clumsy with in the past. In earlier seasons, agents often hint at dramatic personal secrets only to have them resolved quickly in the finale or overshadowed completely by the external threat.
The latest season opens with a bombing rocking a London mall, conducted by a death squad of mercenaries. At the same time, River’s grandfather, a former spy, is attacked in his home, causing River to travel abroad undercover to unearth the links between the terrorism that rocked his city and the violence aimed at his grandfather, only to discover that both hit even closer to home than he thought.
It’s a surprisingly poignant story about nature versus nurture, probing at the hereditary nature of violence. Sure, there are the expected spy-thriller hijinks — higher-ups stabbing each other in the back, hackers toying with surveillance systems and so on — but it is River’s awakening to how his past has shaped him that makes for the most compelling element of the season and the show so far.
Over the past three years, “Slow Horses” has proven that it is one of the most exciting shows currently airing: it follows high-octane seasons with thoughtful melancholy ones, so that ultimately each season has a unique gunshot-to-conversation ratio.
Showrunner Smith understands that it’s important to supplement explosions with exposition and, with each new episode, we grow more attached to our characters as we become more aware of their strengths and vulnerabilities. Some seasons are indulgent and fun in their action set-pieces — the final two episodes of Season 3 see our characters make a thrilling escape from an MI5 facility — while others, like Season 4, are more restrained and touching.
With the next season on the horizon, it is prime time to catch up on a show that is equal parts thrilling and heartbreaking, funny and disturbing — it’s a show that finds the perfect balance between twisty conspiratorial mysteries and the personal growth and transformation of individual characters.
This story has been edited from an earlier version that gave an incorrect first name for author Mick Herron.