A delightful announcement for fans after the dust of the actors’ strike had settled was that Slow Horses Season 4 would premiere a mere ten months after Season 3 dropped on Apple TV+. The darkly funny and intricate espionage series is adapted from the popular Mick Herron Slough House novels, and Spook Street, the source material for Season 4, was an award-winning work.
Slow Horses Season 4 is a more personal and emotionally involved story than previous seasons. It deftly lays the groundwork for the already-greenlit Season 5 and a potential Season 6. Hugo Weaving’s villainous adversary, Frank Harkness, is a chilling addition to the landscape, larger than life and here to stay. As established, the world is full of irredeemable people. Sometimes, only Slow Horses stand in their way.
Season 4 pops us right back into the dynamic dysfunction and complicated relationships of MI5’s misfit toys at Slough House under the direction of Jackson Lamb (Gary Oldman). A major explosion in West London sets off the action at the top end of domestic security, MI5, where Diana Taverner (Kristin Scott Thomas) is once again stewing in Second Desk position, propping up the newly appointed First Desk, Claude Whelan (James Callis).
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Slow Horses, New Faces
Over at Slough House, the post-holidays malaise lingers over our intrepid cock-ups. Roddy Ho (Christopher Chung) is his usual obnoxious self. Shirley Dander (Aimee-Ffion Edwards) and Marcus Longridge (Kadiff Kirwan) alternately confide and conflict over their respective personal challenges. And River Cartwright (Jack Lowden) and Louisa Guy (Rosalind Eleazer) raise a glass.

Joining the Slow Horses are J.K. Coe (Tom Brooke) and Moira Tregorian (Joanna Scanlan). Coe, a tall, lanky, hoodie-wearing ghost of a spook, is a transfer from Psych Eval. Tregorian, used to pulling strings and arranging events at a much more prestigious level, makes the best of things as she settles in as office administrator in the absence of Catherine Standish (Saskia Reeves). (You’ll remember Standish resigned at the end of Slow Horses Season 3 after learning the truth about her late boss, Charles Partner (James Faulkner), and the circumstances of his death.)
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There’s also a new Top Dog in town. Emma Flyte (Ruth Bradley), as the new Head of MI5’s enforcers, is unlike anyone Lamb or any of the Slow Horses have ever come up against. In turn, Taverner’s like no boss Flyte’s ever answered to before.

This Time, It’s Personal
After seasons focused on terrorist cells, sleeper agents, and mercenary teams, Season 4 takes a very personal tack, with a murder attempt on River’s retired MI5 grandfather, David Cartwright (Jonathan Pryce). Season 3 hinted that Cartwright Sr. has begun to have lapses in memory and cognition. At the start of Season 4, River shares with Louisa that his grandfather doesn’t even recognize him at times.
To be fair, it’s not the first time someone’s tried to kill David Cartwright on Slow Horses (never mind all the possible times when he was still an active agent). In Season 3 Episode 6, “Old Scores,” he gunned down a would-be killer at his front door and then invited River in for a cuppa.
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But his declining mental acuity makes him a vulnerable (and problematic) target now. River takes decisive action to figure out the danger’s source. This separates him from the rest of the team for nearly the entirety of the season. Using the attacker’s travel documents, he reverse-track the trail to Les Arbres, an estate in France, where many people don’t appear to like his face.
Tying together the public bombing and the private assassination attempt are the “cold bodies” — cover identities grown over decades by intelligence agencies like MI5 — used by the attackers. Solving the mystery of how these identities were obtained by criminals leads the Slow Horses to some hard truths that hit close to home.
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Breaking the Mold
Season 4 breaks away from some of the tropes we’ve gotten to know in the best possible ways. We still see Lamb and Taverner verbally sparring occasionally on their grudgingly respectful middle ground, but they mostly operate in their separate arenas. Lamb has his dance card full with executing Slough House field maneuvers while having to deal with Tregorian’s compulsive need to organize his office. Oldman exudes Lamb’s signature misanthropic directness with a tad more urgency, especially regarding Tregorian’s meddling.
However, his determination to support River’s operation without being able to contact him belies his callous attitude. Remember, Lamb chose the Slough House job. On some deeply troubled level, he cares for these agents. More than any season before, Lamb puts himself in the fight for his team here. Oldman cannily gives us glimpses of that humanity under all that filth.
Meanwhile, Taverner’s playing puppeteer with First Desk Whelan. We’re so used to seeing James Callis play the smooth manipulator that it’s positively jarring seeing him as a character both naively bumbling and potentially lethal. At first, it seems it has to be an act … but no. He’s just that smarmy.
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At Slough House, Coe quickly becomes our new favorite Slow Horse. He lurks, evades and ignores the others at a level even they recognize as aberrant. On the rare occasion Coe speaks, he is sharply insightful and tactlessly honest. When he takes action, it’s invariably efficient and effective. As his backstory emerges — thanks, Roddy — we begin to suspect that Slough House is both his refuge and his punishment.

Spook Street
Slow Horses Season 4 continues the series’s trademark thrilling, complex, and devastating narrative bent while innovating new developments to keep us on our toes. The season is uniquely focused on River and David’s personal connections to the attackers. As such, several espionage-based plot threads are left dangling along the way. For those of us who like answers to all the questions, the ambiguity of it all leaves the finale feeling distinctly incomplete. One can only hope subsequent seasons will draw the loose ends together in a satisfying way because we’ll definitely be watching.
Slow Horses Season 4 premieres globally on Wednesday, September 4. Then, a new episode weekly, every Wednesday through October 9, on Apple TV+. The first three seasons of Slow Horses are available now on the streaming service.
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