Slow Horsesseason 4, episode 3, “Penny for Your Thoughts,” finally reveals a bit more of the mystery plaguing River Cartwright (Jack Lowden), one ofSlough House’s MI5 agents, and his grandfather, retired MI5 agent David (Jonathan Pryce). Previously,Slow Horsesconfirmed what happened to River. After David killed his grandson’s look-alike and his would-be assassin, River used the dead body to his advantage. Planting his ID at the scene and disfiguring the intruder’s face,River chased his only lead to Lavande, France. Thankfully, Jackson Lamb (Gary Oldman) doubled down on River’s ruse, allowing him more time.

As seen inSlow Horsesseason 4, episode 2, River’s investigation led him to a mysterious compound on the edge of the small town of Lavande. Inside, the massive (and abandoned) house was littered with baby prams and toys as well as weaponry and books about war. A mysterious man (Hugo Weaving) attacked River, though a third party knocked the MI5 agent out and pulled him from the burning compound. Back in London, Kristin Scott Thomas’Diana Taverner indulged in another MI5 cover-up, which would’ve linked the agency to the WestAcres bombing, and blackmailed Claude Whelan (James Callis).

Kristin Scott Thomas as Diana Taverner in Slow Horses season 4 with the members of Slough House behind her

7River Is Being Held Captive By His Look-Alike’s Mother

Bertrand Was Born At The Les Arbres Compound

While other episodes made viewers wonderwhySlow Horses' River went to France, episode 3 finally provides some clarity. Although the show’s many threads haven’t yet formed a clear picture, River’s captors fill in plenty of crucial details. When River comes to, Victor, the man who tailed River to the strange compound in episode 2, levels a gun at the young agent, insisting that someone is going to speak with him soon.That someone ends up being the mother of River’s look-alike, who’s actually named Bertrand, despite what his fake passport might have implied.

Instead, they want to know why River was at Les Arbres — the real name of the compound.

Gary Oldman as Jackson Lamb eating an ice cream and looking confused in Slow Horses season 3 with question marks in the background

When River informs Bertrand’s mother that her son is dead,Victor notes solemnly that, “It was bound to end that way — with a father like that.“Putting two and two together, River pulls out the photograph he lifted from the compound. When River asks Bertrand’s family about the men in the photograph, both of them seem hesitant to give more answers. Instead, they want to know why River was at Les Arbres — the real name of the compound. River reveals Bertrand attempted to kill his grandfather, hence his untimely demise. He explains that his family is in danger, too.

6River Learns More About Frank Harkness

Hugo Weaving’s Character Has A Dark History & Connections To WestAcres

After their uncomfortable introduction, River sits down with Bertrand’s mother and Victor, and they eventually disclose the name of the older man in the photograph: Frank Harkness (Hugo Weaving). Frank is not just the owner of the now burnt-down compound; he’s also the person who tried to strangle River in episode 2. Victor wants to know why Frank was after River, but the MI5 agent insists there’s no larger connection.River believes he got in Frank’s way while he was trying to destroy evidence at Les Arbres. Although River’s new informants don’t seem convinced, they drop the subject.

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Bertrand’s mother explains that she was one of many mothers who Frank Harkness brought to the compound. Evidently,Slow Horsesseason 4’s villain impregnated several women and tossed them out of Les Arbresafter they gave birth. “He makes them leave,” Bertrand’s mother explains, suggesting that Frank wanted to raise the boys on his own. Given the sheer amount of weaponry and warfare-related books River stumbled upon in Les Arbres, all signs point to Frank mustering an army of loyal child soldiers who, of course, are now adult assassins — one of whom is seemingly the WestAcres bomber.

Jackson Lamb on the phone with slow horses characters in the background

New episodes ofSlow Horsesseason 4 premiere on Wednesday on Apple TV+ through July 17, 2025.

While the episode doesn’t really delve into Frank’s motivations — or if the WestAcres bombing is part of his plan or a son gone rogue — it certainly gives viewers much more to chew over. Unfortunately,River’s stay is cut short by a group of murderous Lavande localswho burst into the house. Evidently, Bertrand made many enemies thanks to his ruthless lifestyle. River manages to escape Lavande, but his next move isn’t so clear.

Paul Hobden and Jackson Lamb Slow Horses

5David Cartwright Tells Standish He Needs To Tell First Desk About A “Bomb”

Standish Moves David To A New Location

Back in London, David Cartwright is still lying low at Catherine Standish’s (Saskia Reeves) flat. Between David’s growing paranoia and worsening dementia, the former MI5 head is incredibly restless. While in the bathroom, he mutters about killing Bertrand for River’s sake — to protect his grandson.Slow Horsesdoesn’t reveal what, exactly, David was protecting River from, especially since it seemed like Bertrand was gunning for David. As a former operative, David likely has many enemies. At the same time,David remains an unreliable narrator; as he bursts out of the bathroom and doesn’t recognize Standish anymore.

The frazzled elder Cartwright tells Standish that he needs to inform MI5 about ‘the bomb’…

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Even so, David insists on scheduling a meeting with First Desk. The frazzled elder Cartwright tells Standish that he needs to inform MI5 about “the bomb,” as well as something to do with River. David’s confusion is punctuated by moments of clarity; he’s finally able to place Standish — even if he’s disparaging about it.Standish convinces David to stand down, reminding him that First Desk is in no position to take meetings. Instead, Standish promises to arrange a meeting on his behalf. In the meantime, she moves him to a “safe house”: her vacationing neighbor’s empty apartment.

4Flyte Tries To Arrest Jackson Lamb

Lamb Reveals That Standish Is Housing David Cartwright

At Slough House, Emma Flyte (Ruth Bradley), the new Head of the Dogs, bursts onto the scene to arrest Jackson Lamb. When Lamb arrives at Slough House, he knows Flyte is waiting for him and tells Louisa Guy (Rosalind Eleazar) to contact Standish; David needs to be moved so that Flyte doesn’t find him. Louisa agrees to help and tries to phone Standish. Up in Lamb’s office,Flyte insists on cuffing Lamb and bringing him in for obstructing an ongoing investigation. Lamb says that he didn’t want to blow his joe’s cover, because, clearly, River wanted to be presumed dead.

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Flyte reminds Lamb that River was at a murder scene before fleeing, which doesn’t seem to unnerve Lamb all that much. Instead, the head of Slough House strikes a deal with Flyte, agreeing to tell her David Cartwright’s location if Flyte drops the charges. Lamb reveals that Standish, a former member of Slough House, is harboring David. Meanwhile, Louisa can’t get Standish on the phone and tries her best to obstruct Flyte’s raid.Since she’s already moved David to her neighbor’s flat, Standish is cooperative.Once again, Flyte turns up empty-handed in her pursuit of the Cartwright men.

3Claude Whelan Tasks Giti With Investigating Other Cold Bodies

Whelan Wants To Keep Taverner In The Dark

At the Park, Whelan, who assumed First Desk at the beginning of season 4, decides to take some control back from Taverner. Despite being Second Desk, Taverner is much more well-versed in MI5’s goings-ons than Whelan. Previously, an agent named Giti approached Taverner with some potentially damaging intel: the WestAcres bomber’s passport, under the name Robert Winters, was issued by MI5. Ultimately, Taverner covered up the agency’s connection to the bomber and uses Giti’s investigation to blackmail Whelan. Evidently,Winters was one of many “cold bodies” — ready-made identities used by MI5 during the Cold War.

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Eager to reclaim some ground — and get to the bottom of things — Whelan approaches Giti in episode 4 to task her with investigating other cold bodies. To make matters more complicated,Whelan wants to keep Giti’s work off of Taverner’s radar. At first, Giti is apprehensive; she doesn’t want to get on Taverner’s bad side. At the same time, Whelan is First Desk, so Giti agrees to help him in secret. The extent of Taverner’s cover-up, and even just how much the cold bodies thread will factor into the conspiracy at large, remains to be seen.

2Moira Reveals That David’s Bagman Went To Lavande In The ’90s

Frank’s Assassin Hunts David’s Bagman

In episode 2 ofSlow Horsesseason 4,Lamb strikes a deal with Moira Tregorian(Joanna Scanlan), Standish’s replacement at Slough House. Moira wants out of the purgatory that is Slough House — and, perhaps even more, she wants to escape Lamb’s stench and filth — so she agrees to use her contacts at the Park to delve deeper into David Cartwright’s history. InSlow Horsesseason 4’s third outing, Moira reveals that there’s no evidence of David going to France — thoughDavid’s bagman — essentially a staff officer —didtravel to none other than Lavande, France, during the 1990s.

Like previous outings,Slow Horsesseason 4 contains six episodes.

Realizing the bagman could be in danger, Lamb gets him on the phone. The new assassin, who was last seen planting a camera at David’s house and chatting to Frank on the phone in episode 2, is hot on the bagman’s tail. Frank has instructed this assassin to eliminate David’s former bagman — and then move on to David. With the aid of Lamb and a few other members ofSlow Horses' cast of characters, namely Slough House agents Shirley (Aimee-Ffion Edwards) and Marcus (Kadiff Kirwan), the bagman survives the assassination attempt. Unfortunately,Frank’s man also winds up escaping.

1David Cartwright Disappears

Standish & Louisa Guy Aren’t Sure What To Do At The End Of Slow Horses Season 4, Episode 3

After Flyte leaves Standish’s flat, Louisa bursts onto the scene to ask Standish why she wasn’t answering her phone. The former Slough House operative reveals that River took her phone with him for his international investigation.Standish asks for more intel about the WestAcres bombingwhile Louisa inquires about River’s mission. However, neither character knows much at all beyond the basics. Catherine tells Louisa that the “safe house” she escorted David to is actually her neighbor’s flat, so the two head downstairs. Aside from the neighbor’s cat, the flat is empty — and the backdoor is wide open.

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With no signs of forced entry,it seems likely that David Cartwright fled on his own accord. It’s possible David didn’t trust Standish, who had no intention of setting up a meeting with First Desk and took matters into his own hands. It’s possibleSlow Horsesseason 4, episode 4, will see David arriving at the Park to warn Whelan and Taverner of whatever threats he believes are lurking. Of course, with Frank’s latest assassin still on the move and Flyte eager to bring River’s grandfather in, David Cartwright is in danger from all sides.

Slow Horses

Slow Horses follows a dysfunctional team of MI5 agents led by the notorious Jackson Lamb. Set against the backdrop of the espionage world, the agents must navigate complex operations and defend England from sinister forces, all while dealing with internal struggles and their abrasive boss.