Spy thrillers bring to mind a few things: beautiful women, brushed steel gadgetry and the inevitable “good guy” victory. “Slow Horses” gives these traditional spy thriller tropes the finger, then farts for good measure. Its fourth season, which premiered on Apple TV+ last month, is no exception, unflinchingly spotlighting espionage’s dirty underbelly — plans gone wrong, shifting allegiances and a chilling disregard for human life.

The show, adapted from Mick Herron’s “Slough House” novels, follows the misadventures of a ragtag team of MI5 agents in London relegated to Slough House — a distant, dilapidated office for agents who made critical mistakes in the line of duty. Led by the deceptively shabby Jackson Lamb (Gary Oldman), who emits as many sarcastic one-liners as he does odors, the slow horses are supposed to spend their time doing old paperwork. Instead, they do anything but.

For four seasons in a row, the slow horses have invariably ended up at the center of intelligence fiascos, thwarting high-profile extremist kidnappings, Russian bomb threats and internal sabotage. Their pariah status makes them the perfect pawns for Diana Taverner (Kristin Scott Thomas), the ambitious second-in-command at MI5, who often uses them to do her dirty work.

River Cartwright (Jack Lowden), a slow horse with a tendency for heroics, is all too willing to dive headfirst into danger, tangling himself in complicated criminal plots. He’s joined by unbearable computer whiz Roddy (Christopher Chung), short-tempered Louisa (Rosalind Eleazar), shorter-tempered former coke addict Shirley (Aimee-Ffion Edwards) and chronic gambler Marcus (Kadiff Kirwan), among others.

But in season four, River becomes a critical piece of the plot itself. The first episode opens with River’s grandfather David (Jonathan Pryce), a former MI5 agent suffering from cognitive decline, committing an unthinkable crime in self-defense. Over the next few episodes, the slow horses work to uncover David’s complicated history. Eventually, River finds himself face-to-face with American mercenary Frank Harkness (Hugo Weaving), who holds the keys to his past, as MI5 closes in with a shoot-to-kill order.

Drenched in blues and grays, the show’s cinematography highlights the grittiness of urban London. Blood spatters and car crashes are careless and commonplace. Its real triumph lies in the performances of its protagonists, who carry the show deeper into personal plotlines for the first time, despite a weaker script.

Oldman is at once repulsive and endearing as Lamb, but where his character has been a constant driving presence in the past, this season, he’s conspicuously absent from the screen. Instead, Lowden takes center stage as River, and while his familial angst is a captivating story on its own, the narrative felt a little lopsided compared to previous seasons. Unrecognizable behind a bushy beard, Weaving valiantly attempts to fight down his British accent to play Harkness, but his character is so utterly evil that at times, it feels two-dimensional.

Traditionally a bottle show, “Slow Horses” leaves the viewer with multiple open-ended questions at the end of season four — perhaps an indicator of a pivot to longer-form arcs after its renewal for two more seasons.

Filming is complete on season five, with episodes anticipated to be released in the summer of 2025. After the blood’s cleaned up and the dust settles, the slow horses are always, somehow, ready for more. So far, viewers are too.