Slow Horses Season 4 Ending Explained: Frank Harkness, River Cartwright & What’s Next?

Warning: contains major finale spoilers.

Usually, January is a time for enforced sobriety and regretting the post-Xmas overdraft. Not for the Slow Horses. They spent their Jan trying to protect River Cartwright’s grandfather from a trained assassin, and sadly, getting caught in the crossfire. 

Season four was a family affair – specifically, River’s family. River met his long-lost father – a psychopath mercenary who ran a death squad made up of his own biological sons – and found out that he had three half-brothers, none of whom survived the season. River was also forced to put his beloved grandfather into a home, and the Old Bastard didn’t go willingly. The Slough House family suffered losses too, but just about remains standing. Let’s dig in to the finale’s twists and revelations. Final spoiler warning…

Why Did They Let Frank Harkness Go?

MI5 let the man who was indirectly responsible for the Westacres bombing and for countless other international assassinations (not to mention a recent spate of murders around London and a grenade in Regent’s Canal) walk free because… he was on their payroll. Not in any official capacity, of course, but in an under-the-table arrangement.

The letters Frank left with Molly Doran were addressed to the intelligence service Heads of Department, and contained details of past jobs his death squad had carried out for British Intelligence. Frank threatened that if he wasn’t released by a certain time, the letters would be published online, thereby incriminating the service in a series of illicit  assassinations that could cause a meltdown in international relations. So they let him go. 

As Jackson tells Taverner, David ‘The Old Bastard’ Cartwright may have told Harkness to piss off when he first offered his death squad’s services, but David’s successors clearly didn’t do the same. 

What Happened to the “Psycho Gulf Prince”?

In episode four, Frank was threatened with dismemberment by the client who’d hired his squad to carry out the unspecified assassination that became into the Westacres bombing. (We still don’t know who the target of that assassination was.) Frank’s son Yves was supposed to kill one man in the shopping centre car park, but instead blew himself and 23 shoppers up in a suicide attack. The Gulf prince wasn’t happy and told Frank that if he didn’t kill the two men who could link him to the attack – David Cartwright and Bad Sam Chapman – by midnight, then he’d be a goner. 

Patrice did kill Bad Sam, but thanks to the Slough House team, he failed to kill David Cartwright. No matter, because Frank’s blackmail terms included MI5 getting the “psycho gulf prince” off his back. They presumably did, and he was free to walk.

Why Was Frank Trying to Kill Chapman and Cartwright?

This was explained several times, but just in case you’d missed a bit by popping to the loo or letting the dog in from the garden, let’s recap: 30-odd years ago, Frank Harkness pitched his death squad’s services to David Cartwright, who turned him down. In retaliation, Frank seduced Cartwright’s young daughter Isobel, making her part of his harem at Les Arbres and impregnating her (with baby River, who was raised by his grandparents David and Rose). 

To get his his daughter back, Cartwright sent Sam Chapman to Lavende with a car filled with MI5 “cold body” IDs, cash and explosives to trade for Isobel’s return. Because Frank’s son Yves used one of these MI5 IDs to carry out the Westacres attack, Frank knew that David and Sam could trace it back to him, and so he sent his other sons Bertrand and Patrice to kill them for their silence. It was a clean-up job, in short, necessitated by Yves going off piste and committing a mass atrocity instead of the political assassination he’d been hired to carry out.

What Next for Frank Harkness?

All of his sons, bar River, are now dead but still, he said he planned to “rebuild” his organisation. He might be getting on a bit to try a repeat of his original plan, which involved impregnating a load of young women, stealing their babies and raising them abusively to become ultra-loyal super-killers, but Frank seems resourceful. In fact, he does reappear in a couple of books’ time in Mick Herron’s Slow Horses novel series…

What Happened to Marcus Longridge’s Debt?

He paid it off. Very luckily, Marcus was able to turn the £3k he’d got from selling his service weapon into enough money to buy back his gun and pay off his £10k debt through a last-ditch gamble. Very unluckily, he then had his head blown off by Patrice Harkness while trying to protect his colleagues at Slough House. Marcus’ family home is safe from his debtors now, and thanks to Jackson Lamb, his family are going to receive a decade’s worth of his salary as a death-in-service payment. 

Why did Whelan Authorise a ‘Shoot to Kill’ on River?

In the first decisive act of Whelan’s dithery ‘can’t you do it, mum?’ tenure, he overruled Taverner on the shoot-to-kill because he feared that River might be part of the terrorist cell behind the Westacres bomb. Not knowing whether River was a victim or an accomplice of Patrice and Frank, Whelan would rather gamble on wrongly killing one man than allowing another Westacres to happen should River prove to have turned to the dark side. Story-wise, his decision also ramped up the finale peril nicely.

Is Catherine Standish Back Now?

She certainly seems to be. Jackson Lamb never accepted her letter of resignation, and this whole David Cartwright business dragged her back into the beating heart of Slough House. Moira Tregorian successfully blackmailed Claude Whelan about his kerb-crawling record and got her old job back at the Park, leaving room for Catherine’s return.

Who’s “Karla”?

A little nod to a character from the world of spy author John Le Carré. When David Cartwright is protesting about not being able to protect himself and needing to be left in a retirement home, he asks, who’s supposed to be coming after him, and jokes “Karla?” Karla is a Soviet intelligence officer who’s a recurring character in the Le Carré universe, and the antagonist of George Smiley. 

When Is Slow Horses Returning?

We don’t have a date, but season five has already been filmed and so is currently being prepared. Expect it to arrive in 2025.

Most Important of All…How Did Standish Know Where Jackson Lamb Kept His Handcuffs?

We’re sure she’s just very organised. Nothing to see here.

Slow Horses season 4 is available to stream on Apple TV+.