Slow Horses Season 6 Ending Explained: Here's What Happens To River

This article contains major spoilers for "Slow Horses" Season 6, Episode 6, "Judgement Day."

The best spy show on streaming just ended with a massive cliffhanger.

After surviving a hellish conflict with Russian agents and a reunion with his mercenary father (Hugo Weaving) at the end of "Slow Horses" Season 6, River Cartwright (Jack Lowden) learns that his seemingly resurrected fellow agent Sidonie Baker (Olivia Cooke), in whom he's romantically interested, has outwitted her pursuers, made her way to River's late grandfather's house, and "left a light on" for River, calling back to a sweet moment the two of them shared earlier.

It seems like the season is going to end on a high note. When River walks into the house, he hears that Sid is taking a shower upstairs. He prepares the kitchen for a romantic evening, making a pot of tea and tidying up the kitchen a bit before ultimately deciding to switch from tea to wine. He lights  a candle and makes sure it's placed on the table just so. It's a cute moment — he's nervous, and you can tell he's excited to finally have the latest chaotic adventure behind him and get some quality time with Sid.

But as he attempts to open the wine bottle, River suddenly falls to his knees, breathing quick and shallow. He seems to choke as he tries to call out Sid's name, which she can't hear over the running water. He collapses to the floor, tries to pull out his phone, we see his foot spasm ... and that's the end of the episode.

I was initially confused about what exactly happened in that moment, but a scene from earlier in the season explains everything.

In the beginning of Season 6, Episode 3, "Resurrection," as Cartwright is learning the details about how Sid survived being shot in the head back in Season 1, a pair of Russian assassins, credited as "Jim" and "Jane" (Kyle Soller and MyAnna Buring, respectively), arrive at River's grandfather's house to kill Sid. While Jane chases River and Sid across the property to a neighboring farmhouse, Jim briefly stays behind in the kitchen. He pulls out a duffel bag, removes a box labeled "sore throat spray," and meticulously lays its contents on the table in plastic bags, along with some heavy duty rubber gloves. Then he looks around the room, and the camera rack-focuses to the crowbar River brought inside at the end of Episode 2, when he thought he might need it to fight his own father. 

That's all we actually see on screen, and then the show moves on at its typical breakneck pace, with Jim and Jane tracking River, Sid, and Coe (Tom Brooke) to a nearby bowling alley, and Coe dying in a brutal hand-to-hand fight with Jim. "Slow Horses" has never been sentimental about its deaths, and that stays true in Season 6.

The implication from that quiet moment in Episode 3 is that Jim put some kind of poison on the crowbar, hoping that River or Sid would pick it up at some point. In real life, Russia has a long history of using poison for assassination attempts; within the past few years, Vladimir Putin's government used a chemical weapon called Novichok to try and kill his political rival Alexei Navalny, which was documented in an Oscar-winning 2022 documentary. (Navalny eventually died in a Russian prison.) 

It's not clear exactly what type of poison Jim is using here, but River falls prey to it in the finale episode when he's tidying up the kitchen and moves the crowbar off the counter. He immediately notices something is odd about the crowbar, because he looks back at it and then down at his hand after touching it, and dramatically wipes his hand off. As the scene progresses, you can see him flexing his fingers multiple times as the poison starts to take hold.

I totally forgot about Jim quietly skulking around the late David Cartwright's (Jonathan Pryce) kitchen in Episode 3, not only because his actions aren't explicitly shown, but because so much else happens in "Slow Horses" Season 6. This show has always been action-packed, but this season had such a momentum and heft to it that it almost felt as if it was building to a genuine ending of the series as a whole. (Don't worry, it's renewed through at least Season 8 already.) 

In addition to Coe's death, it seemed as if former slow horse Louisa Guy (Rosalind Eleazar) might bite the dust as well, and former "Dog" Emma Flyte (Ruth Bradley) actually did meet her end after a heroic stand on the beach. The scope of this season is pretty huge — Diana Taverner (Kristin Scott Thomas) is compromised by a right-wing news personality, River and Sid stage an explosive escape from a high security facility, Roddy Ho (Christopher Chung) nearly dies but is saved by a child. I wonder if the series can keep upping the stakes from here, or if, like my beloved "Fast & Furious" franchise after "Fast X," it might be better to scale back in its next entry instead of trying to top itself.

We'll see what direction the show decides to take, but for now, even though River and Sid teamed up and took Jim down in brutal fashion by that lake in Episode 5, the Russian agent may have gotten the last laugh.

All six seasons of "Slow Horses" are now streaming on Apple TV.