'Slow Horses' season 4: What happens when an old spy gets confused?

“Slow Horses” returns on Apple TV+ and the misfits and losers of Britain’s MI5 domestic counterintelligence agency — collectively known as the slow horses, a sneering nickname that speaks to their perceived uselessness — find themselves working a case yet again.

This time it involves their fellow reject River Cartwright and his far more respected grandfather, the former head of MI5. Once sharp, the old man has become disoriented lately, and when a visitor arrives at his quiet rural home, he greets them with the business end of a shotgun. Blood is spilled and the cavalry is called.

Was it all a big mistake? Or is something more sinister going on connected to his bygone days on the job? The slovenly Jackson Lamb, the exquisite Diana Taverner and the assorted slow horses must figure it out.

Based on Mick Herron’s Slough House book series — named for the dingy London headquarters where the slow horses have been banished — Season 4 adapts the 2017 novel “Spook Street.” It begins with a bang, as David Cartwright (Jonathan Pryce) blows away someone he believes has infiltrated his home. Who the hell did he just kill?

Lamb (Gary Oldman) arrives and, with typical unemotional disinterest, IDs the body. Chances are, he’s lying about whose corpse lies splayed in that bathtub. It’s a choice that has all the hallmarks of the simple but necessary subterfuge that is Lamb’s stock in trade.

Meanwhile, a car bomb has exploded in London and Taverner (Kristin Scott Thomas, formidable as ever) is tasked with finding out what happened and preventing any further incidents.

One of the long-running jokes of the series is that, as MI5’s No. 2, the top job remains forever just out of reach. That means she’s stuck answering to intellectually inferior men and she can’t help but cop an attitude in her own pristine way.

But it’s never clear what drives her. Does she actually care about preventing carnage and something as squishy as … human lives? “There isn’t a big picture to running an intelligence agency,” she sighs, “it’s just putting out fires every bloody day.” Maybe she’s just obsessed with the job and the power it confers.

Somehow the car bomb and that death in David Cartwright’s home are connected, which necessitates a sojourn to France, where someone has tried to raise a small army of killers from birth. For what purpose? Unclear. But this ragtag paramilitary operation has fallen apart now that its members have grown into adults.

What remains are just a few thugs, but their leader (Hugo Weaving) has an important connection to old man Cartwright and lingering resentments have a way of, well, lingering.

Weaving is especially good as an entirely realistic villain, playing him with an American accent and an American sense of entitlement. It is a wonderfully grounded contrast to his similarly nefarious Agent Smith from “The Matrix” franchise. A more complex performance, too.

If the show’s third season was unusually obsessed with guns, the violence here erupts with more thought and narrative purpose and it doesn’t overstay its welcome.

As a series, “Slow Horses” doesn’t offer tightly plotted, clockwork spy stories; think too deeply about any of the details and the whole thing threatens to fall apart. But on a scene-by-scene basis, the writing is such a delicious combination of wry and tension-filled, and the cumulative effect is wonderfully entertaining. Spies have to deal with petty office politics like everyone else!

Even so, I remain unconvinced the show knows what to do with its various slow horses. Outside of River Cartwright (Jack Lowden), who is intense and droll, they are too one-dimensional to justify their screen time. The rancid charisma of Lamb (who seems slightly less putrid this season; he’s still a greasy mess, but the dark overcoat he wears pulls him together in a way that his rumpled raincoat never did) and Taverner’s wily gamesmanship do much of the heavy lifting.

Oldman and Thomas are the kind of seasoned performers who bring real vitality to their knives-out dynamic, which more or less repeats itself each season. That’s not a complaint. “Slow Horses” doesn’t pretend that the series or its characters need to evolve in order to remain interesting.

Tackling a new case each season, while keeping the same format and framework, is enormously satisfying when done well. And it’s one of the few shows that has avoided the dreaded one or two year delay between seasons that has become standard for streaming. Instead, it provides the kind of reliability that has become increasingly rare. It probably helps that each season is based on one of Herron’s books.

A consistent theme in “Slow Horses” is that the younger generation — even the non-screwups at The Park, Slough House’s upscale counterpart — aren’t especially good at this spy stuff.

At least, they’re no match for the cagey instincts and hard-won experience of Lamb and Taverner and anyone else who cut their teeth during the Cold War. It’s not that the old guard are invulnerable, they’re just smarter somehow. The newer generation? One bad guy manages to pull off an ambush that thwarts all their training.

Herron and the show aren’t just cynical about MI5’s corruption, they’re cynical about the agency’s ability to do anything even remotely resembling the job at hand.