The spy novel, writes Dame Stella Rimington, former chief of Britain’s Home Security Service, MI5, and since her retirement herself a novelist, is ‘in a special class of literature in which the real and the imaginary can be mixed in any proportion, so long as both are present’. The operative phrase is here ‘in any proportion’. Most successful literature grafts some measure of the real onto the imaginary. The spy genre is especially marked by its demotic appetites and expansive tastes. It can manage a potentially endless range of human situations and can adjust to all manner of plot speeds and density. In some truly great spy novels, force, vim and sheer chutzpah make up for the evident brutality of the construction; in the greatest of them all, atmospheres of menace, gloom and terror are summoned up in the most elegant and refined manner.

Above all, spy fiction makes hay from the picky detail, the world put just a little out of kilter. In John le Carré’s masterpiece Tinker Tailor Soldier Spy, a relatively junior operative, Ricky Tarr, realises that betrayal is afoot after he sends a workaday telegram back to MI6 headquarters. Unpredictable consequences ensue, which draw him deep into networks of betrayal and moral compromise. A flaw in his reality is thrown wide open and the adventure begins. Towards the climax of the novel, a tiny, almost casual, deceit leads to the unmasking of Bill Haydon as the Soviet ‘mole’ at the heart of the Service. Haydon comes into the office to deal with the fallout from the shooting of an agent behind enemy lines in Czechoslovakia, informing a rather startled duty officer that he had been alerted by the ticker tape at his gentlemen’s club. But the news broke in the middle of the night when no club is open; Haydon must therefore have been forewarned. It is this broken thread that reveals a tapestry of lies and betrayal. In John Buchan’s The Thirty-Nine Steps, Richard Hannay is propelled into a thrilling manhunt up and down the British Isles when a neighbour comes knocking at his door. What if he hadn’t been in? He might never have known. The neighbour winds up dead shortly after his introduction.

Mick Herron is the latest anglophone writer to captivate audiences with the world of spies and spying. His Slow Horses novels, now running into their fourth season on Apple TV+ in Will Smith’s brilliant adaptations, have received rave reviews. The final episode of the series aired this week. And don’t worry, fans: if one novel = one season, then there are several more left. And Herron has given no indication that he is about to stop writing novels based on the same imaginative territory and cast of characters. Herron’s ‘slow horses’, rejects from the MI5 headquarters, The Park, are sent to Slough House as a punishment for past misdemeanours. They’re stuck in the MI5 gulag precisely because of the picky details: River Cartwright, the most plausibly glamorous and Bond-like of the troupe, is (wrongly) accused of incorrectly ID-ing a terror suspect as part of an MI5-run training exercise, causing the mass evacuation of King’s Cross Station. There’s a mix-up – was he told to look out for a blue tee under a white shirt or a white tee under a blue shirt? He gets it wrong and professional purgatory ensues. It is later revealed that River has been framed: a Service rival had deliberately given him the wrong ID. Why? He had noticed a picky detail – on another exercise, he clocks something he really shouldn’t have seen: a meeting between Diana Taverner, one of MI5’s top people, and an undercover agent engaged in an illegal mission conceived by Taverner herself.

The bedfellow of the spy novel is the crime novel, full of characters who follow up on the picky detail, who cannot let it slide. In the Danish police drama, The Killing, Sarah Lund is a cop for whom the picky detail is everything. She wakes up in the middle of the night. She can’t rub it out of her mind until she gets a good explanation and, potentially, a new lead. Many of Herron’s plots follow this logic, occasionally beginning with seemingly trivial details. Often, Herron displays a taste for the spectacular: a seemingly motiveless terror attack in a busy shopping centre; the untimely death of a central, and much loved, character. The novels adapt well to screen, because they run with a great sense of discipline. Not a word is spared that does not build into the action – in each novel, the weather is nearly always the same throughout: it is either snowing, or raining, or really hot. In Joe Country, much of the action takes place in the frozen English countryside (just one of the rare trips beyond London for the crew), and the atmosphere resembles the celebrated episode of The Sopranos, ‘Pine Barrens’, which serves up a similarly tragi-comic mix of mistaken identity and ultra-violence, prismed by a frozen landscape. Herron’s cultural hinterland is far more diffuse than simply the classic rollcall of English spy novelists.

River is brought up by his grandfather, David, an ex- MI5 chief. He confesses to Sid, another of the slow horses: ‘I grew up with the stories.’ He means the fictional world of the spy novel: ‘The first bedtime story he ever did read me was Kim… After that, well, Conrad, Greene, Somerset Maugham… For my twelfth birthday, he brought me le Carré’s collected works. I can still remember what he said about them: They’re made up. But that doesn’t mean they’re not true.’ Herron slyly subverts this view: yes, they might have been true once, but not in modern Britain. In Herron’s novels, when River goes searching for parallels from the Cold War, he finds himself stuck in metatextual dead ends – whenever River does attempt to ape the fictional spymasters of his boyhood imagination, he makes major misjudgements or reads the scene wrong. Playing the hero gets him into a heap of trouble.

Herron’s world does not reflect either the derring-do of the late-Victorian spy novel, nor the shattered certainties of the British Empire in decline, nor the murky moral equivalences of the Cold War. River and his fellow slow horses must confront a new reality. The new monsters are corporate greed, the soft hypocrisies of HR culture, generic incompetence, and a British state whose institutions, though fragile, are capable of astonishing ruthlessness when their interests are challenged. Very little of the Slough House series takes place abroad. There’s a brief sojourn in France, captured in season 4 of the television adaptation, a couple of snapshots of Russia, and Jackson Lamb’s fragmentary memories of Berlin. Almost all of the UK action takes place in London and its periphery. Herron delights in the crappiness of contemporary Britain. One scene shows River stuck on the Tube, an interlude in a harum-scarum dash across London. Extreme peril and thrilling chases play out in abandoned parking lots, across dreary suburban sprawl and, occasionally, in absurdly bucolic Home Counties idylls, populated by rich commuters and second homers. But for Herron, the capital is everything, the perfect place for a spy out to settle old scores.

Herron’s hero is Jackson Lamb, played to perfection by Gary Oldman. Herron himself clearly doesn’t quite know how to handle him: unlike many of the other characters (most of all River, through whose eyes the reader often sees the action), we only get one fleeting glimpse from Lamb’s viewpoint: in the opening pages of Real Tigers, the second book in the series, Lamb struggles to put his socks on, then stumbles around his office, momentarily confused, and that’s it. He eludes his own author. And yet Herron’s most striking and suggestive images are reserved for Lamb. Although grotesquely unfit, he also moves with unnatural speed. He is at home in the half-light. His only apparent moral commitment is to go the ends of the earth for his ‘joes’. He’s a spy’s spy. He has no personal ambition. He breaks all the rules to protect his agents in the field. He has a cause, and he is utterly faithful to it.

What makes the perfect spy? John le Carré’s Magnus Pym, protagonist of his great semi-autobiographical novel A Perfect Spy, captures something of the paradoxes of his trade: ‘We betray to be loyal. Betrayal is like imagining when the reality isn’t good enough… Betrayal as hope and compensation. As the making of a better land. Betrayal as love. As a tribute to our unlived lives… Betrayal as escape. As a constructive act. As a statement of ideals. Worship. As an adventure of the soul. Betrayal as travel: how can we discover new places if we never leave home?’ The perfect traitor/spy is the ultimate alchemist, turning mud into gold, lies into truth and back again.

The Renaissance philosopher Michel de Montaigne writes, in his essay On Liars: ‘If a lie, like truth, had only one face we could be on better terms, for certainty would be the reverse of what the liar said. But the reverse side of truth has a hundred thousand shapes and no defined limits… Only one flight leads to the bull’s eye: a thousand can miss it.’ The modern spy novel has a unique complication in that its subjects are both trained in deceit and revel in it. They are professional liars, capable of fictionalising their own timeline into unlimited potential storylines. This propensity to spin reality into unlikely and novel forms goes far beyond assuming false identities, cover names or fake moustaches. That lies have many thousands of shapes, and the truth only one, is convenient for the spy and the spy novelist – they have infinite canvases to make their own. ‘It would be beautiful in another context,’ muses the spymaster George Smiley, after he uncovers Bill Haydon’s betrayal and the devices he had used to deceive his colleagues. That ‘other context’, in Tinker Tailor Soldier Spy, is not only the history of art, but of the novel itself: we thrill at the near-perfection of Haydon’s treacherous tradecraft perhaps a little more than we are appalled by it.

Herron’s world works, both on page and on screen, not just because he is a great storyteller, or has a keen grasp of the mechanics of espionage, but because he simply gets an awful lot right about why modern Britain is the way it is. In Slough House, there are no bureaucrats. There are skilful, brilliant agents doing incredibly boring, pointless bureaucratic work. But they know that what they are doing is pointless; and that forces the crew to go out on adventures. They go about it with devil-may-care tenacity, and (spoiler alert) a quite alarming number of them end up making the ultimate sacrifice.

Once the slow horses have something proper to do, away from ‘home’, they prove themselves, again and again, as savvy, streetwise and astonishingly competent. Most British institutions are full of such people – but they rarely flourish and make it to the top. The administrative class, dedicated first and foremost to its own survival, tends to promote employees in its own image: the smooth talkers and backbiters who always get the tone right and never put a single nose out of joint. One of Herron’s most brilliant creations, ‘First Desk’ Claude Whelan, expertly translated to the screen in Season 4, is amply representative of this class. In contrast to the slow horses, such bureaucrats are absolutely committed to self-preservation and institutional loyalty. Lamb is loyal to his joes, not to the ‘blob’.

Jackson Lamb is indeed the perfect spy, and in a decidedly imperfect (and perfectly recognisable) Britain, that gives him a certain moral gravitas, even in this most dubious of trades. That is the genius of Mick Herron’s creation: he does what the spy novelist has always done – dare to widen the cracks in the placid edifice of the everyday to see what lies beneath. Throughout the Slough House series, horror, extreme behaviour, turmoil and terror tend to dominate the scene. But you will also find excitement, bravado, personal loyalty, sadness and love. The slow horses are modern Britain’s perfect spies – and that’s why we, fans of the books and the TV series, keep wanting more.