In the rural isolation of north Devon, Max Janáček, a retired intelligence operative living quietly in a centuries-old cottage, is roused from insomnia by an intruder forcing open his kitchen window. Max strikes the woman unconscious, discovers she carries a Taser but no identification, and realizes she is not alone. With his landline cut and his mobile useless, he flees into the dark, pursued on foot and by motorbike through hedged lanes and fields. Using local knowledge and improvised obstacles, he evades his attackers. At dawn he retrieves a flight kit hidden beneath a floorboard, containing a false passport, cash, and disguise materials, then collects a hidden second car and drives away.
The narrative shifts backward to explain how these events began. Two years earlier, a British Prime Minister, seeking revenge against Regent's Park, the headquarters of the intelligence services, for reporting behavior that cost him his position as Foreign Secretary, established the Monochrome inquiry to investigate historical overreaching by the intelligence services. First Desk, the head of Regent's Park, immediately defanged the inquiry by exploiting a loophole: She denied the panel physical access to the Park and its archive and required all file requests to follow a deliberately labyrinthine cataloguing system.
Monochrome's daily operations fell to Griselda Fleet, a Black woman in her fifties seconded from the Home Office to lead the inquiry, and Malcolm Kyle, her anxious 32-year-old subordinate. Griselda had been coerced into the role by the PM's special adviser, who implied she would lose her job if she refused. The inquiry heard mostly useless testimony, and the panel members, including the elderly Sir Winston Day as president and Carl Singer, a businessman with government contracting ties, treated it as a paid inconvenience.
Everything changes when a classified Regent's Park file is secretly slipped into Malcolm's shopping trolley at a supermarket. The file documents a Berlin Station operation from 1994 involving operatives named Alison North and Brinsley Miles. After a sleepless night and a fruitless appeal to his former boss, Malcolm brings the file to Griselda at the Monochrome offices. They agree to treat it as legitimate inquiry material and distribute copies to the full panel.
First Desk works to shut the inquiry down. She offers the Chancellor documents revealing that Fabian de Vries, a wealthy businessman, has been secretly funding the former PM's legal costs. In exchange, the Chancellor agrees to block De Vries's bid for the government's vetting services and to terminate Monochrome.
In London, Max seeks help from Shelley McVie, a former handler from Housekeeping, the Park division that provides care and support to retired operatives. Through Shelley, Max tracks his current handler, John Bachelor, to a pub, where the woman from his cottage reappears with a companion and tries to seize him. Bachelor and Shelley intervene, and Max interrogates the woman. She reveals she works for Four Corners, a private security subsidiary of Singer Industries, whose CEO is Carl Singer, the same businessman sitting on the Monochrome panel.
Before the shutdown order arrives, the panel convenes to hear witness #137, who states her work name as Alison North. Her testimony transports the narrative to Berlin in 1994. She was sent as a junior Housekeeping trainee, ostensibly for a compliance review. In reality, David Cartwright, a powerful behind-the-scenes figure at Regent's Park, secretly tasked her with observing Brinsley Miles, Berlin Station's most experienced operative. Cartwright suspected Miles was compromised and wanted material to use as leverage.
Miles was crude, chain-smoking, and provocative, but brilliant. He took Alison to strip clubs and dive bars, where she met Otis, a charismatic German who was Miles's closest friend, with deep roots in Berlin's post-reunification underworld.
Alison uncovered the real story behind Miles's activities. Years earlier, Miles had run a female officer of the Stasi, East Germany's secret police, code-named Bogart, as an asset behind the Berlin Wall. After the Wall fell, someone learned the mole was a woman but could not identify which one. All three female officers of the appropriate rank were hanged with piano wire in a forest. Miles received a photograph of the atrocity showing a man standing among the bodies. This man, Karl Schenker, became Miles's obsession.
Miles had diverted Park funds to help Otis purchase a house in East Berlin, spreading word that it contained duplicate Stasi records that would compromise former officers, hoping to lure Schenker from hiding. Alison realized Otis himself was the bait. Despite promising Miles time before contacting London, she reported everything to Cartwright immediately. The operation collapsed. Otis privately revealed that his sister was not Miles's asset but one of the other women hanged alongside Bogart.
Days later, Otis took Alison to collect his repaired Chevrolet Camaro. She sat in the driver's seat, turned the ignition key, and a car bomb detonated: "her life divided in two" (338).
A text message interrupts the testimony: The Home Office has discontinued Monochrome. The panel scatters, but Sir Winston left without formally closing proceedings, so Griselda, as acting president, reconvenes privately with Malcolm. Alison reveals that the mole Miles uncovered was Charles Partner, who later became First Desk. Cartwright's true purpose was never to investigate Miles but to gather compromising material that would force Miles to assassinate Partner, a task too politically dangerous for normal channels. Alison then reveals she is Molly Doran, the archivist of Regent's Park, who uses a wheelchair as a result of the bombing.
In the present day, the full scope of Molly's scheme emerges. Her assistant Erin Grey, researching the government's Green Shoots privatization initiative, discovered a photograph of De Vries. Molly recognized him as Karl Schenker from a violent encounter in Berlin decades earlier. She had Erin slip the OTIS file to Malcolm, knowing De Vries maintained a spy on the Monochrome panel: Griselda herself, who had been accepting payments. The file's surfacing would reach De Vries, and its reference to Otis, now living under the name Max Janáček, would compel De Vries to act against Max, thereby exposing himself as Schenker.
First Desk pieces the scheme together and sets her own trap. She has Griselda call De Vries to a safe house, where First Desk waits. After a seemingly conciliatory negotiation, De Vries excuses himself to the bathroom, his coffee having been doctored with a diuretic. He finds a Russian-made Makarov pistol on the toilet lid, picks it up, and emerges holding it. First Desk screams and throws her hands in the air. Her personal security officer, stationed across the street, shoots De Vries through the eye. First Desk frames the killing as self-defense.
In the novel's final scene, First Desk sits in her back garden with a visitor: Brinsley Miles, still in the Service. Miles reveals the last piece of the puzzle: Cartwright had deliberately told Charles Partner about the Berlin trap, knowing Partner, as the mole, would relay it to Moscow, which would warn Schenker. The operation's catastrophic failure left Miles so compromised he had no choice but to assassinate Partner. Miles stands, stares down the garden as if remembering another life, and departs.