Mick Herron’s Slow Horses was published in 2010 and is the first novel in Herron’s Slough House series of spy thrillers, which includes eight novels and several associated novellas. Slough House pays homage to many of the conventions of the spy thriller novel while simultaneously subverting them. Set in a specifically British setting and drawing on particularly British spy novel tropes and sardonic humor, Slow Horses explores the nature of second chances, personal motivation, and heroes and antiheroes.

Slow Horses was longlisted for the Ian Fleming Steel Dagger Award 2010. It was adapted as a series for Apple TV+ in 2022, for which Herron was jointly awarded the USC Libraries Scripter Award 2023.

This guide uses the 2020 Soho Press paperback edition.

Content Warning: The source material contains portrayals of racist and sexist attitudes and language, and expletives that this guide makes reference to. Both the source material and the guide contain material relating to alcohol dependency, extremist terrorist activity, kidnapping, threat, violence, violent death, death by suicide, and gory detail.

Plot Summary

Slow Horses follows the story of Jackson Lamb and his team of agents, all of whom have been sent to work at Slough House, an offshoot of MI5, after being exiled from the Regent’s Park HQ for various offenses. Known derogatively by MI5 as “slow horses,” these agents are considered expendable and given tasks that are tedious, morally questionable, or considered too reputationally or physically disastrous for HQ to take responsibility for.

River Cartwright fails his training simulation exercise: If it had been a live operation, King’s Cross Station would have been bombed. For this mistake, he is sent to Slough House to join the slow horses. Run by Jackson Lamb—who is eccentric, rude, and living with an alcohol dependency—these Slough House agents work at desks in professional obscurity, enduring insults from their boss. River feels particularly humiliated because his grandfather, David Cartwright, is a legendary retired spy.

When a young man is taken hostage by a terrorist group who threaten to behead him within 48 hours, the slow horses know that their services will not be requested. This does not prevent River from investigating the situation further. One of his fellow agents, Sidonie “Sid” Baker, has been following a disgraced journalist, Robert Hobden. Hobden’s extreme right-wing views have exiled him from mainstream journalism and politics, and River notices a connection between Hobden’s rhetoric and the group’s name, “Voice of Albion.” While everyone else has assumed that the terrorists are an offshoot of an overseas Islamic extremist group, River discovers that they are a right-wing UK organization with extremist and racist anti-Arab views. Their hostage, a British Pakistani man named Hassan Ahmed, personifies their racist white-supremacist beliefs that British Muslims are a threat to “traditional” British society.

River is instructed by Diana Taverner, MI5’s “Second Desk” (the second in command), to take Hobden’s laptop to MI5 HQ in Regent’s Park. There he meets with James “Spider” Webb, whose faulty information caused River’s mistake during training. River is not authorized to open the laptop but he does so, copying the files. When he looks at them, he finds only the number pi to thousands of decimal points. He wonders what Taverner wants with such files.

Meanwhile, another of the slow horses, Jed Moody, meets with Taverner. He tells her he has installed in Lamb’s office and suspects that Lamb is running a secret operation out of Slough House. Sick of his tedious work and Lamb’s rudeness, he wants to please Taverner, with the aim of being reinstated at HQ. Taverner instead tells him he is to assassinate the journalist Hobden. It becomes increasingly clear that Taverner is somehow involved in the hostage situation. She tells Moody, for example, that she was the one who assigned Sid Baker to follow Hobden. She also instructs Moody to remove the bug; she does not want to raise Lamb’s suspicions.

River surveilles Hobden’s house, convinced that Hobden is connected to the kidnap. Sid surprises him there. She has been clandestinely following him and reveals that she has not been banished to Slough House like the others. Rather, she has been sent there to watch River. Before he can question her, they see a masked man entering Hobden’s yard. In the ensuing chase, Hobden escapes, and the unknown assailant flees after attacking River and shooting Sid in the head.

River wakes up in the hospital, where he is detained by “the Dogs,” the internal security branch of the Service. Sid has survived and is recovering. While he awaits questioning by Regent’s Park, Lamb shows up and frees River. Lamb suspects that River is being scapegoated. Two other slow horses, Louisa Guy and Min Harper, go to the pub. They are curious about what Lamb knows about the hostage situation and drunkenly decide to snoop in Slough House after hours. They break in but instead of investigating they begin to kiss, until they are interrupted by someone creeping about in Lamb’s office. In the scuffle that follows, Min and the masked intruder tumble down the stairs. The intruder dies in the fall. When Min lifts his mask, it’s Jed Moody.

When Lamb and River arrive on the scene, Lamb tells Louisa and Min to get their stories straight. He pulls a burner phone from Moody’s pocket and dials the number. Diana Taverner answers, and she and Lamb meet in secret. She confesses that she has concocted the hostage situation. She has a secret agent inside the Voice of Albion. Taverner’s plan is to rescue the hostage and generate good publicity for the Service, bolstering her career. She admits it is not going as planned: The escaped Hobden might know her scheme—he overheard her speaking to a politician at a bar. It also seems the hostage, Hassan, is related to one of the directors of Pakistan’s intelligence service. If he is killed, there will be international repercussions.

Taverner knows where the terrorists are keeping Hassan and suggests that Lamb send in his slow horses to rescue him. She presents this as an opportunity for the disgraced agents to redeem themselves. Lamb is suspicious, but he agrees, on the condition that nobody else gets harmed in the process. Unbeknownst to them, however, Hobden has approached Conservative politician Peter Judd and together they have leaked the information that MI5 is behind the kidnapping. The kidnappers—Larry, Curly, and Moe—now know that one of them is a secret agent. Curly realizes that this is Moe—really called Alan Black—and kills him. When the slow horses arrive at the scene, they find the house empty and Black’s severed head on the kitchen table.

Lamb knows that he and his team have been set up by Taverner to take the blame for her failed plan. He tells River, Louisa, and Min to gather the other slow horses and meet him at Blake’s grave. He himself will fetch Catherine Standish, the Slough House office manager. While two of the slow horses are intercepted by the Dogs, Louisa and Min manage to retrieve Roderick Ho, their computer expert. Lamb and Standish are intercepted by the Dogs, but Standish—usually mild-mannered—pulls a gun. When they rendezvous, River tells Lamb that he has proof of Taverner’s involvement in the conspiracy, so the two of them leave for Regent’s Park. The others will try to rescue Hassan, whose whereabouts are now unknown. Even though Lamb rudely dismisses their skills, Standish believes that he wants them to try.

Lamb confronts Taverner: Because of her actions, two agents have been killed and another has been injured. Taverner has already lied to her boss, blaming Lamb and his team. Pretending retribution, Lamb distracts Taverner and her agents by faking a bomb threat in MI5 while River meanwhile infiltrates the building. While breaking in, River retrieves the file on his training exercise, kept by Spider. He finds that Taverner wanted River exiled to Slough House because River had trailed her as part of another training assignment, and unknowingly photographed her with Alan Black. He now has evidence that Taverner deliberately ruined his career, as well as proof that she was behind the hostage incident.

In the interim, Catherine Standish and the others have tracked the location of the Hassan: Epping Forest, outside of London. Curly intends to decapitate Hassan with an axe and post this online. Curly and Larry start arguing, and Hassan runs away. Curly follows, and a struggle with the axe ensures. Hassan gains the upper hand but decides not to kill Curly and walks away. Hassan find the police on the road nearby, alerted by the slow horse.

In the aftermath, four of the slow horses are gone: the two who were taken by the Dogs; Jed Moody, who died; and Sid Baker, who has disappeared from the hospital. There are no records of her at the hospital or anywhere else. River keeps her barrette at Slough House as evidence she once existed. Louisa and Min’s relationship romantic relationship is growing. The novel ends with Lamb contemplating his future. Perhaps he is not as ready to step away from the action as he'd thought.