Revelations about a protagonist’s origins are one of the most engaging ways to expand a series’ mythology, especially when they upend long-held understandings about the character. It expands and pivots our understanding of the hero, casting a new light on everything we thought we knew. Think of the famed reveal of the villainous parental legacy of Luke Skywalker (Mark Hamill) in Return of the Jedi or the secret kingly heritage of Strider/Aragorn (Viggo Mortensen) in Peter Jackson’s The Lord of the Rings series. These sorts of secrets are the lifeblood of the famously twisty spy thriller subgenre, like how we discover in Skyfall that the Christoph Waltz interpretation of James Bond arch-nemesis Ernst Blofeld is Bond’s foster brother. It’s about time we had one for Slow Horses, one of the best espionage series of our era, and its protagonist, River Cartwright (Jack Lowden). In Season 4, the hidden histories of the Cartwright family are revealed, creating an engaging and tension-ridden hunt for the truth.

What Is ‘Slow Horses’ Season 4 About?

Season 4 of Slow Horses starts with a literal bang: a bomb goes off, and the man who triggered it is discovered to be in use of a ready-made MI5 secret identity. Needless to say, that’s a problem for MI5’s new First Desk, Claude Whelan (James Callis), who struggles wildly to contain the aftermath in one of the series’ plotlines. Simultaneously, River comes to discover that his grandfather David Cartwright (Jonathan Pryce), an MI5 legend with declining mental health, has been targeted for assassination. It all sets off a deepening series of threats and investigations, and at the center of it all is Frank Harkness (Hugo Weaving), a mysterious and deadly antagonist that may be the series’ most dangerous thus far. The various plotlines congeal in the end, in classic Slow Horses style, building towards a grand finale that has huge implications for the slow horses and for River himself.

Season 4 is essentially about chickens coming home to roost, the sins of the past becoming an undeniable force in the present, both for major characters and for the agency itself. Slow Horses often throws curveballs and balances competing plotlines, and it’s never shy about revealing a new facet of MI5’s shady underbelly, all factors that work extremely well this time around. It makes for an exciting season of action and reveals that shine new light on many subtle elements of past plot while finding fresh possible terrain for the future. Slough House may be an administrative purgatory for MI5’s muck-ups, embarrassments, and failures, but Slow Horses keeps finding new life and fresh possibilities in the wreckage.

‘Slow Horses’ Season 4 Is a Tense Outing Full of Dark Secrets
Image via Apple TV+

Season 4 strikes a strong balance between dangerous situations and moments of well-constructed situational comedy. Gary Oldman continues to expertly deliver Jackson Lamb’s sardonic wit alongside the occasional proud flatulence and copious amounts of alcohol. When one of the slow horses appears to have shuffled off this mortal coil (something the team regularly, and regrettably, deals with), computer expert Roddy (Christopher Chung) humorously wastes no time in claiming their computer. When the season pivots back to Slough House, the team’s interactions are full of moments like these, which land well and give color and charm to a tense mood. That said, while the team comes to the fore more as the story progresses, the bulk of Season 4 hovers around the Cartwrights, River and David, and their connection to a set of dark, long-hidden secrets.

The high point of the season, however, comes with the deepening investigation into the assassination attempt on David Cartwright’s life. David effectively raised River, so there are high emotional stakes for the series’ protagonist, and Lowden lands River’s isolated seriousness and the emotional stakes well every time he’s onscreen. Pryce additionally gets his fair share of screen time in this season, and brings a lot of pathos to his character’s moments of confusion, frustration, and desire to do right by River. The season’s antagonist, Frank Harkness, is a harrowing threat in charge of a truly disturbing network of assassins and mercenaries, and Hugo Weaving unsurprisingly excels at bringing intensity and menace to the villain. It isn’t the first time that the slow horses have seemed out of their depth, but Season 4 really does build a sense of unnerving danger thanks to Weaving’s presence.

While the series’ central mystery provides for an engaging journey and a great exploration of the Cartwrights’ past, the season’s biggest flaw is that the rest of Slough House often feels too tangential to much of the proceedings. There are excellent moments for Louisa (Rosalind Eleazar) and some other members of the team, but for the most part, they seem properly tertiary, sidelined in favor of the explorations of David and River’s secretive backstory. While the latter makes for excellent television, the interactions between the slow horses are always welcome elements of every new chapter. The season seems a little unbalanced as a result, at least until things ramp up for the team towards the end. To some degree, this issue’s a natural consequence of a series having only six episodes a season — certainly something has to give when new, twisty subplots emerge. Still, Season 4 does at times feel like there’s slight difficulty this time in balancing the various threads. Nonetheless, it’s an exciting installment of a show that clearly hasn’t exhausted its ability to surprise and delight.

Strong Additions To the Cast Propel ‘Slow Horses’ Season 4 To Greater Heights
Image via Apple TV+

Altogether, Slow Horses’ excellent Season 4 reveals that there are many unexplored layers of depth behind MI5 and these characters, creating the groundwork for a compelling fifth season. Oldman remains delightful as ever as our favorite ornery boss, while Lowden handles the season’s complexity with aplomb. Pryce is excellent as well as the plot rotates heavily around his prior years in MI5, and Weaving’s dramatic heft hangs over the team as a heavy threat. It’s easy to believe that Frank Harkness is 10 steps ahead of the team, and the resultant feeling that danger is inevitably closing in on the team serves the tension well.

Beyond the drama, Slow Horses hasn’t lost the ability to find irony and comedy in the middle of a bad situation, and fans of the series won’t be disappointed. There’s a little difficulty in fitting the new threats and characters into the existing world in a balanced way, and it does at times feel like plotlines are left lingering too long. Quite a few of the slow horses are often unfortunately sidelined here, a feeling that’s inescapably highlighted every time we revisit the team, but it remains a great season overall with some stellar surprises for new and returning viewers alike.

Slow Horses Season 4 premieres on Apple TV+ on September 4.

Watch on Apple TV+