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A close-up of Jackson Lamb in Slow Horses Season 4, Episode 6

Slow Horses Season 4’s Ending, Explained

Warning: The following article contains spoilers for Slow Horses Season 4, Episode 6, “Hello Goodbye.”

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The sixth and final episode of Slow Horses‘ fourth season contains enough narrative twists and turns to make even an MI5 agent’s head spin! But fret not: we’ve explained all of the biggest plot developments from Slow Horses Season 4’s ending below.

How Is Harkness River Cartwright’s Father?

River Cartwright and Frank Harkness in Slow Horses Season 4, Episode 6

Well, the “how” seems kind’ve obvious (wink, wink), so we’ll reframe the question: How did Frank Harkness know River Cartwright’s mother, Isobel? This is already covered earlier in Season 4; Isobel was one of several women Harkness seduced so that he could father a squad of mercenaries trained from birth. Harkness targeted Isobel because her father, David Cartwright, was a high-ranking member of MI5. But it’s not until Episode 6 that we get explicit confirmation of what many of us suspected: River is one of Harkness’s kids ā€“ only raised in the UK by David and his wife Rose, not Frank.

What Was In Harkness’s Letters (& Who Were They For)?

While Frank Harkness threatens to kill MI5 records keeper Molly DoranĀ in Slow Horses Season 4, Episode 5, he ultimately leaves her with a handful of envelopes, instead. In Episode 6, we find out that these envelopes are addressed to Claude Whelan and the rest of British intelligence’s top brass. What’s inside? Letters detailing the years’ worth of not-so-savory activity Harkness and his mercenaries have carried out on MI5’s orders. They’re literally Frank’s “get out of jail free” card (more on that later).

Why Does Whelan Give Tregorian Her Job Back?

As is so often the case in Slow Horses Season 4, Episode 6, the answer here is “blackmail.” In Episode 5, Moira Tregorian learns that Claude Whelan exiled her to Slough House for stumbling upon evidence that the MI5 boss regularly engages prostitutes. Tregorian would’ve realized sooner, except she didn’t know that Whelan’s Met Police codename is “Galahad.” Now aware that Whelan and Galahad are one and the same, Tregorian uses her newfound leverage over ol’ Claude to reclaim her old job at the Park, as well as a four-day week and pay bump!

Related: How To Read the Slough House (Slow Horses) Books in Order

Why Does MI5 Let Harkness Go?

Claude Whelan and Diana Taverner in Slow Horses Season 4, Episode 6

As noted above, Harkness circulates a bunch of incriminating letters to British intelligence’s most senior people, however, that’s only the first part of his contingency plan. The second part is that Harkness has scheduled an email containing the letters’ contents to hit media outlets’ inboxes if he’s not set free in 12 hours. So, the choice facing Whelan, Taverner, and the rest is simple: release Harkness or publicly admit to employing his death squad. Unsurprisingly, they run with the latter option.

What Does Lamb Make River Sign in the Pub?

Slow Horses‘ fourth season wraps up with a scene between Jackson Lamb and River Cartwright in a pub. Before the pair silently share a drink, Lamb passes River a document to sign without reading. If that sounds dodgy, that’s because it is ā€“ but it’s also designed to keep River out of trouble! This crumpled scrap of paper is Lamb’s fudged account of River’s movements throughout Season 4, which reframe his self-guided exploits as an official operation on behalf of Lamb and Slough House.

Related: All Major Actors & Cast List for Slow Horses Season 4

Is There A Post-Credits Scene?

No, Slow Horses Season 4, Episode 4 doesn’t have a post-credits scene. However, it does include a trailer for Slow Horses Season 5! So, don’t exit to the Apple TV+ menu after we fade out on Lamb and River’s quiet moment together in the pub, or you’ll miss out.

Slow Horses Season 4 is currently streaming on Apple TV+.


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Image of Leon Miller
Leon Miller
Leon is a freelance contributor at The Escapist, covering movies, TV, video games, and comics. Active in the industry since 2016, Leon's previous by-lines include articles for Polygon, Popverse, Screen Rant, CBR, Dexerto, Cultured Vultures, PanelxPanel, Taste of Cinema, and more.