TL;DR: Slow Horses Season 4 cranked up the chaos with Frank Harkness revealed as River’s father and ultimate villain, Marcus’s heartbreaking death, and a finale that left nearly every storyline dangling. It’s messy, it’s devastating, and it’s exactly why this misfit spy show is still Apple TV+’s best-kept secret.
Slow Horses
There’s a very particular kind of joy that comes from watching Apple TV+’s Slow Horses, a joy that sneaks up on you. On paper, it’s just another spy thriller. On screen, though, it’s the most human, cynical, foul-mouthed antidote to all those glossy Bond imitations that mistake slickness for soul. By the time Season 4 wrapped, I realized I’d stopped treating it as just a show and started treating it like a weekly hangout with the most dysfunctional coworkers in the intelligence business. Which, honestly, is the real magic trick Slow Horses keeps pulling off: it gets me to care about these burnouts, has-beens, and screwups even more than the big spy games they’re entangled in.
Season 4, though? It didn’t just turn up the dial—it snapped it clean off. The season was both a brutal gut punch and a devilish tease, ending on a cliffhanger that left me pacing the living room muttering about Frank Harkness like he was some neighbor who kept dumping trash in my yard. And now, with Season 5 about to drop, I’ve been replaying every jagged moment, every loss, every sideways insult from Lamb, and trying to piece together where this grimy little family of spooks goes from here.
The season kicked off with a literal bang: a bomb tearing through a London shopping center. That alone would’ve been enough to keep me glued to the couch, but the investigation quickly spiraled into one of the nastiest revelations the series has ever dropped: Frank Harkness. Hugo Weaving, in full menacing mode, brought to life a villain so oily and calculating he made the Russian agents of past seasons feel like Saturday morning cartoon baddies. And here’s the kicker—he’s River’s dad. That reveal landed like a knife twist, the kind of personal betrayal that elevates a spy story into something Shakespearean. The sins of the father aren’t just haunting River—they’re actively blowing holes in his life.
Harkness isn’t just some terrorist-for-hire. He’s the sort of Big Bad who plays at family values while casually weaponizing his own children. One son turned assassin, another cannon fodder, all bent to his master plan of raising a mercenary army. River, poor River, had to hunt him down only to realize the monster at the center of it all shares his DNA. The chase sequences were tense, sure, but it was the quieter horror—the visit to Molly’s home, the lingering sense of menace—that really sold Harkness as a villain. The fact that he’s still out there at the end, protected by the secrets he dangled in front of Diana Taverner, means Season 5 has a ticking time bomb of its own baked right in.
But while Harkness loomed large, the real heartbreak of the season came from within Slough House. Catherine Standish never made it back, and her absence was a wound the team couldn’t quite cover. Even Jackson Lamb, ever the expert in masking sentiment behind a haze of cigarette smoke and insults, felt her loss. And then came the death that broke me: Marcus Longridge. Of all the Horses, Marcus was the one I’d always relied on to bring a bit of levity, a steadying presence amid the chaos. Watching him gunned down by one of Harkness’s sons in their own damn office was brutal, a reminder that even in this band of misfits, no one is safe.
Shirley’s grief was palpable. She and Marcus had forged a bond, one built on shared struggles and rough-edged banter, and losing him wasn’t just losing a colleague—it was losing one of the few people who saw her fully. Add in her ongoing battles with addiction, and you can see the storm clouds gathering over her arc in Season 5. It’s the sort of loss that doesn’t just sit quietly in the background; it gnaws, it festers, and it changes the way you move through the world. I’m worried about Shirley, and that’s testament to how well the show has developed her from comic relief into something far rawer.
The finale, though, was where Slow Horses twisted the knife. For once, the show refused to tidy things up. No neat bow, no sense of closure. Harkness walked free. Diana was still stuck under Claude Whelan’s smug incompetence. Catherine remained adrift. And River, who’s endured more than any golden-boy washout deserves, had to make the most devastating call of all: placing his grandfather in assisted living. Jonathan Pryce made that scene unforgettable, his anguished repetition of “You promised me” echoing like a curse as River walked away. It was almost too much—and then Lamb did the unthinkable. He invited River for a drink. No snark, no scorn, just a small gesture of respect. In Lamb’s world, that’s a hug, a confessional, and a lifetime achievement award rolled into one.
And so here we are, dangling at the edge of Season 5. Will Diana finally topple Whelan and seize MI5’s crown? Will Shirley’s grief drag her deeper into the abyss? Will River untangle the mess of his family legacy, or just drown in it? And will Roddy Ho finally, mercifully, find a girlfriend who isn’t catfishing him? (Let’s be real: probably not.) The genius of Slow Horses is that I don’t know—and I don’t care if the show takes its sweet time answering. As long as Jackson Lamb keeps snarling insults from behind his greasy curtain of hair, I’ll keep watching.
Slow Horses remains Apple TV+’s most underappreciated gem, a spy thriller that swaps glamour for grit and finds its heart in the broken people the world writes off. If you’re caught up through Season 4, you know the stakes have never been higher. And if you’re new? Buckle in. It’s not just the explosions that’ll get you—it’s the way these losers somehow become the people you’d trust most when everything goes to hell.
Season 4 of Slow Horses is as bleak, brutal, and brilliant as the series has ever been. By tying River’s personal history to its most dangerous villain yet, the show raised the stakes without losing the grubby humanity that makes it special. We lost Marcus, we lost Catherine, and River lost a piece of his heart, but we gained one hell of a setup for Season 5. This is spy drama at its rawest and most relatable.
First two episodes of Season 5 will premiere on September 24.
