Mick Herron’s 2025 novel Clown Town is the ninth installment in his acclaimed Slough House series of satirical spy thrillers. The story revisits the “slow horses,” a team of disgraced MI5 agents relegated to a dilapidated London office building, as they become entangled in a high-stakes conflict rooted in the intelligence service’s dark past. When a secret audio recording from a legendary spymaster’s library resurfaces, it triggers a dangerous chain of events, forcing the ruthless head of MI5, Diana Taverner, into a desperate plot to neutralize both the historical threat and a corrupt politician who is blackmailing her. The novel explores themes of The Moral Compromise Required in Espionage, Institutional Self-Preservation Over Justice, and The Inescapable Weight of the Past.

Herron is an award-winning British author widely celebrated for his contributions to the spy fiction genre. His Slough House series has achieved both critical acclaim and commercial success, earning him multiple Crime Writers’ Association (CWA) Dagger Awards, including the prestigious Gold Dagger for the novel Dead Lions. The series has reached a broad international audience through the popular Apple TV+ adaptation, Slow Horses, starring Gary Oldman as the slovenly but brilliant spymaster Jackson Lamb. The television show has been recognized with numerous accolades, including several British Academy Television Awards (BAFTAs), cementing the series’s place as a significant work of contemporary espionage fiction.

This guide refers to the 2025 Soho Crime edition.

Content Warning: The source material and guide feature depictions of graphic violence, cursing, substance use, addiction, and death.

Plot Summary

At Slough House, an administrative dumping ground for disgraced MI5 agents, the so-called “slow horses” endure mind-numbing tasks under their slovenly but cunning boss, Jackson Lamb. River Cartwright, an agent on medical leave after a near-fatal Novichok poisoning, learns from Oxford researcher Erin Grey that a book from the library of his late grandfather—a legendary spy nicknamed “the O.B.” (for “old bastard”)—is not only missing but also appears never to have existed. The only evidence of this phantom book, The Secret Voices: A Hidden History of Deep Cover Lives, is a brief video River took of the library before the book’s disappearance. Meanwhile, Diana Taverner, the head of MI5 known as First Desk, finds herself in a precarious position. She is being manipulated by corrupt former politician Peter Judd, who secretly financed some of her past covert operations with money provided by the Chinese state—information that would be highly damaging to her and to MI5 if it became public. Taverner receives an anonymous email containing a scratchy audio recording that gives her an idea for a counter-move. She meets with Lamb, telling him that River will be medically discharged, and asks Lamb’s tech expert, Roddy Ho, to identify an employee who has filed an HR grievance against her. Lamb suspects she is manipulating him. Elsewhere, slow horse Louisa Guy considers a lucrative job offer from Devon Welles, a former MI5 internal security “Dog” now in the private sector, which would allow her to escape her dead-end career.

River travels to Oxford and learns from Erin that the non-existent book was actually a box safe. Erin suspects it was taken by Charles “CC” Stamoran, an elderly former agent who helped her organize the library. River confronts Stamoran, who admits to taking the box but claims it contained pornography that he destroyed to protect River’s grandfather’s reputation. River is unsettled but later realizes this is a lie. The real secret is that Stamoran is the leader of a defunct team of Cold War-era spies called the “Brains Trust,” consisting of himself, Avril Potts, Al Hawke, and Daisy Wessex. Stamoran reveals to his team that he is blackmailing the Service with a secret recording he found in the box-safe. The tape contains details of Operation Pitchfork, a long-ago covert operation in which MI5 knowingly allowed the sadistic Irish Republican Army enforcer Dougie Malone to continue committing brutal murders in order to preserve their access to information. He has already sent an extract to Taverner, demanding compensation for their ruined careers.

Pressured by both Judd’s demands and Stamoran’s blackmail, Taverner devises a complex scheme. She contacts River’s partner, Sid Baker—a former agent also on medical leave after being shot in the head—and offers to reverse River’s termination if Sid acts as her go-between with Stamoran. Taverner gives Sid £1000 and a burner phone to deliver to Stamoran in an Oxford safe house. When Sid arrives, she is met by the entire Brains Trust. Daisy, triggered by the unexpected intrusion, attacks Sid with a knife. The team takes the money and phone, then abducts Sid before abandoning her at a motorway service station, taking her personal phone in the process. River, unable to reach Sid, tracks Stamoran to the empty safe house, where he hears Sid’s phone ringing inside. He and Louisa Guy, who has driven to Oxford to help, break into the house, triggering an alarm. They are confronted by Diana Taverner’s enforcers, known as “the Dogs,” and after River punches one of them, they flee in a chaotic car chase.

Taverner, now in contact with Stamoran via the burner phone, arranges a meeting. She reveals her trump card: She knows about the Brains Trust’s involvement in Operation Pitchfork, and she knows that they murdered Dougie Malone. This secret was known only to River’s grandfather, who had scribbled it in the margins of their personnel files, which Taverner has since erased. She uses this knowledge to coerce Stamoran into assassinating her tormentor, Peter Judd, promising to protect his friends if he complies. She reminds Stamoran that he is terminally ill and thus has little to lose. Meanwhile, the slow horses learn from Devon Welles that Judd will be at the Nob-Nobs nightclub that evening. Fearing an assassination plot, the slow horses race to intervene, unaware of the full scope of Taverner’s plan. The Brains Trust, suspecting Stamoran is walking into a trap, tracks him to the club, as do the two Dogs whom River and Louisa previously escaped.

All parties converge on the darkened nightclub. Inside, River and Sid confront Judd just as Stamoran arrives with a gun. Avril and Al enter and try to de-escalate the situation. The tense standoff explodes into chaos when another slow horse, Ashley Khan, accidentally flips a switch that activates blinding strobe lights. The disorienting effect triggers a massive stroke in Stamoran, who collapses. His gun skitters across the floor, where it is retrieved by Al Hawke. As Judd shoves Daisy to the ground in his attempt to flee, an enraged Al aims the gun at him. Louisa tackles Al just as he fires, and the stray bullet hits Ashley in the chest. In the same moment, Daisy recovers, rushes to Al’s defense, and stabs Louisa in the throat with a concealed blade. In the aftermath, Ashley and Louisa are left in critical condition, while Al and Daisy vanish.

Seeking revenge for his fallen agents, Lamb orchestrates a complex counter-plan. First, he meets with Taverner and bluffs, leading her to believe he knows nothing of her true scheme. He has already manipulated Devon Welles into providing the “kill code” for Taverner’s home security by falsely telling him that Judd was responsible for the death of their mutual friend and former colleague, Emma Flyte. He then tasks Roddy Ho with disabling the extensive CCTV network around Taverner’s Notting Hill home, creating a surveillance blackout. Under the pretense of arranging a peace summit, Lamb has River drive Peter Judd to Taverner’s now-unsecured house.

Lamb is waiting inside. He confronts Judd, revealing his knowledge of Judd’s financial ties to China and his manipulation of Taverner. Lamb then shoots and kills Judd using Taverner’s own pistol, which he retrieved from her safe after Welles used the kill code. He wipes the gun and leaves it in the open safe, perfectly framing a now-vulnerable Taverner for the murder. Taverner is arrested, her career destroyed. Back at the hospital, Ashley awakens from surgery, but CC dies from his stroke. The remaining slow horses, including a shaken Roddy who was accosted by a mob while disabling the cameras, are left to process their losses. Sid confronts River over his complicity in Lamb’s plan, insisting that she wants both of them to leave intelligence work behind before it kills them. Catherine Standish, having narrowly avoided a relapse of her alcohol use disorder amidst the trauma, returns to Slough House to find Lamb at his desk, as if nothing has happened. He confirms that he has brought about Taverner’s downfall, telling Catherine he can read her “like a […] [b]ook” (337).