Spoilers are ahead for Slow Horses season 4's ending.

In the River's connection to Frank out in the open, the two men trade verbal blows. Frank desperately wants River to leave Slough House for mercenary life. Meanwhile, James Whelan (James Callis), the ever-incompetent First Desk, orders a "shoot to kill" directive on both Frank and River.

Louisa Guy (Rosalind Eleazar) calls upon Roddy Ho (Christopher Chung) to trace River's phone. One grenade and a chase on foot later, River tracks down his father, allowing Flyte to finally arrest Frank. Once he is in custody at the Park, Frank reveals his insurance policy to Diana Taverner (Kristin Scott Thomas). In short, Frank is prepared to leak MI5's incriminating, decades-long connection to his hired guns. Meanwhile, at Slough House, Marcus Longridge (Kadiff Kirwan) dies protecting David Cartwright (Jonathan Pryce) from Patrice. After everything, River makes good on his promise to bring David to an elder-care facility.

What Happens To Frank Harkness In Slow Horses Season 4’s Finale

Frank Is Taken To The Park But Blackmails MI5

At the beginning of Slow Horses season 4's final episode, Frank Harkness and Patrice seem to have the advantage. Patrice escorts his half-brother, River, to an upscale bar where Frank is waiting. The estranged father-son duo finally have the chance to speak face-to-face — and it doesn't go very well. Although he tried to kill River in Slow Horses season 4, episode 2, Frank insists that River should his dwindling ranks. River asks a few questions about his mother, Isobel (Anna Wilson-Jones), but mostly confirms the details of Frank's years-long child soldier scheme.

Frank its that his plan, which unfolded at Les Arbres, did what he intended: it bred a handful of loyal-to-a-fault soldiers. River points out that one of them went mad and declared war on Frank — hence the blunder at WestAcres — but Frank isn't too perturbed. In fact, he's more preoccupied with catching his train out of town. When Louisa, Flyte, and a bunch of MI5 operatives descend on the waterfront restaurant, Frank holds River at gunpoint. Louisa and Flyte insist that River is a hostage, despite Whelan's "shoot to kill" order, which causes mass confusion.

If Taverner doesn't release him, Frank has ensured that MI5's connection to his assassins will leak.

During the chaos, Frank tosses a live grenade into River's hood and flees the scene. Louisa manages to toss the explosive into the nearby river, ensuring that no one is hurt. At the train station, River manages to track down his father by leaning into his counter-intelligence instincts. Frank hands over his weapon and lets Flyte take him into the Park. During an interrogation, Frank tells Taverner that he won't be in her custody for long thanks to his insurance plan. If Taverner doesn't release him, Frank has ensured that MI5's connection to his assassins will leak.

How River Cartwright Deals With The Fallout Of His Slow Horses Season 4 Discoveries

Frank Asks River To Him As A Mercenary

Since Slow Horses season 4, episode 1, the adaptation of Mick Herron's Spook Street has put River Cartwright through the wringer. A case of mistaken identity and an attempt on his grandfather's life pushed River to discover his long-buried family secrets. River's mother, Isobel, had been one of the many women that Frank impregnated at Les Arbres to raise his team of child soldiers-turned-loyal-mercenaries. In order to secure Isobel's safety, David, then MI5's First Desk, had given Frank weapons, money, and a bunch of "cold bodies" — MI5's ready-made Cold War-era identities.

Essentially, David enabled Frank's "deniable assassination squad" in order to guarantee River and Isobel's safety. River only really makes the connection between his mother and Les Arbres because he recognizes the compound's mural from the birthday cards he received from her as a child. To make matters worse, Frank preys on River's vulnerabilities. Although River holds his own against his estranged father, Frank definitely knows how to get into the Slough House agent's head. Frank praises River's counter-intelligence abilities, saying that he works well under pressure. Even so, River shows no signs of wanting to Frank's insidious ranks.

Who Dies In The Shootout At Slough House?

Slow Horses Season 4 Body Count Is Surprisingly Low

While Louisa and River are dealing with Frank, the rest of Slough House's agents handle Patrice. Thinking that Sam Chapman (Sean Gilder) will want to know that David is safe at Slough House, Moira Tregorian (Joanna Scanlan) leaves a voicemail on Chapman's phone. Unfortunately, Patrice took Chapman's phone after brutally murdering David's former bagman. Armed with the knowledge of David's whereabouts, Patrice launches a one-man assault on Slough House. To make matters worse, Marcus sold his MI5-issued weapon in order to pay off some lingering gambling debts.

Just before Patrice bursts into the building, Marcus manages to retrieve his gun. Even so, Marcus and J.K. Coe (Tom Brooke) are the only adequately armed of the team. Catherine Standish (Saskia Reeves) barricades the door to Jackson Lamb's (Gary Oldman) office, hiding David and an injured Roddy in the bathroom. Meanwhile, Moira struggles to load a revolver for Shirley Dander (Aimee-Ffion Edwards). In true Slough House fashion, it's an absolute mess. Sadly, Marcus dies defending his team from Patrice, who's eventually taken down by Lamb and Shirley.

All episodes of Slow Horses season 4 are streaming on Apple TV+.

Although every death in Slow Horses is a major blow to the good guys, the loss of Marcus hits Slough House particularly hard. When Lamb leaves the building to meet up with Molly Doran (Naomi Wirthner) — Frank previously entrusted Molly with his "insurance policy" in episode 5 — J.K. kills Patrice to avenge Marcus. Somehow, everyone else makes it through the episode relatively unscathed.

River's Decision To Put David Cartwright In A Facility Explained

River's Grandfather Still Feels Guilty About Hiding His Knowledge Of Les Arbres

After surviving the ordeal with Frank, River decides to make good on his promise to Louisa from season 4's first episode. Instead of avoiding David's worsening dementia, River checks his grandfather into a community living facility. David balks at the idea of living in a "hotel," but River insists that it's a "home." Even so, David feels betrayed by his grandson and assumes that River is doing all of this because David never told him the truth about Frank or Les Arbres. River insists that isn't the case at all, but it does little to mend their duo's fraught dynamic.

Why Moira Blackmails Whelan In Slow Horses Season 4

Taverner Might Completely Sink Whelan’s Chances Of Continuing As First Desk

In a previous episode, Moira, one of the Queens of the Database, enlisted Roddy Ho's help to learn more about who banished her to Slough House. When it became clear that Claude Whelan was the culprit, Moria and Roddy did some additional digging. Roddy was able to discover that Whelan, who went by the code name "Galahad" during his metropolitan police days, solicited a sex worker while on the job. Moira leverages this secret to her advantage in the finale when she shows up at Whelan's office to demand her old job back — with a pay raise.

How Slow Horses Season 4’s Finale Set Up Season 5

Slow Horses Season 5 Will Adapt Mick Herron’s “London Rules”

Undoubtedly, Slow Horses' cast of characters who lives to see the end of season 4, it's likely he'll be back in a future outing. A running theme is that MI5 has messed up as many times as Slough House's agents. Although the agency has plotted its fair share of successful cover-ups, it's hard to imagine that won't come to a head soon.