River and Louisa arrive at a derelict industrial estate west of Ealing as evening falls. Louisa believes they were followed, though River doubts she could have spotted a professional tail. Lamb calls to warn River that Tearney is orchestrating events and will betray them when convenient. He orders them to let Donovan and Traynor take the Grey Books without interference. Soon, Donovan and Traynor emerge from a nearby building.

At Slough House, Lamb orders Ho to drive to a farmhouse near High Wycombe, where Catherine is being held. Meanwhile, Marcus and Shirley, recently fired by Lamb, argue in Marcus’s SUV before speculating that the Grey Books contain damaging state secrets that Donovan plans to leak. They decide to follow River and Louisa to the facility.

Donovan confirms to River and Louisa that Catherine will be released once they retrieve the files, downplaying his team’s role in Monteith’s death. He outlines the plan: River and Louisa will enter first to verify the facility is secure.

Unbeknownst to Donovan and the Horses, Duffy is watching the meeting from a nearby vantage point. Earlier that day, Tearney told him that Donovan’s team had gone rogue and killed Monteith, and she ordered Duffy to eliminate Donovan after he retrieves the files. Duffy agreed to act without official Service backup, seeing an opportunity to restore his standing after conflicts with Taverner.

As Ho drives Lamb out of London, Lamb mocks Ho’s car and identity while chain-smoking. Near the farmhouse, Lamb takes some loose change and walks alone toward the house, instructing Ho to wait 20 minutes before acting.

Inside the factory, River and Louisa find a drain cover leading to an underground chamber. They descend and discover a rotary phone, which rings immediately. An electronically disguised voice verifies their Service credentials via a hidden camera; River and Louisa bluff that they are on a classified Scott-level mission. This response unlocks a hatchway in the floor.

At the farmhouse, Catherine confronts the wine bottle left by Bailey. After struggling with temptation, she breaks the seal but ultimately pours the wine down the sink, refilling the bottle with water to use as a weapon.

River and Louisa descend into the facility’s control center and meet Douglas, a nervous man who has worked alone there for three years. The room contains banks of CCTV monitors showing the complex’s corridors and warehouse spaces. River and Louisa convince Douglas to admit Donovan and Traynor.

After a run-in with a police officer, Shirley accesses the Service files on Donovan and gets Marcus’s attention with critical intelligence: Alison Dunn, the woman killed in Donovan’s car crash, was Traynor’s fiancée and had filed a redacted report after serving with Donovan on a United Nations committee. They spot a Black Arrow van heading toward the estate.

In her office at Regent’s Park, Tearney reflects on how Donovan’s actions have given her leverage against Judd. She visits Taverner’s office to question her about invoices for relocating archived files. Taverner confirms that the job covered materials up to Virgil classification, a designation for second-level secrets where Tearney is known to hide sensitive information. Back in her own office, Tearney realizes her original brief excluded Virgil-level files and suspects Taverner of manipulation. She calls Duffy with new orders.

Donovan and Traynor descend into the facility. Traynor reveals that he is armed and orders River, Louisa, and Douglas to stay put while Donovan searches for the files. Black Arrow operatives suddenly appear on the CCTV monitors. Traynor forces Douglas to identify the operatives’ location and then rushes to warn Donovan. Douglas escapes through the hatch while River and Louisa follow Traynor deeper into the facility.

Duffy watches as the Black Arrow team arrives and deploys amateurishly. He has told them that Donovan murdered their boss, ensuring they will attack aggressively, while his own handpicked team of Dogs waits at the hatch exit. Tearney’s revised orders are clear: eliminate everyone, including the Slough House agents.

In the warehouse archive, Donovan frantically searches files while Traynor barricades the doors. Donovan explains that he is looking for Virgil-level records connected to Tearney within the Grey Books, and he reveals his true motive: Taverner recruited him for this operation to destroy Tearney, framing it as revenge for Alison Dunn’s death. Traynor agrees to defend their position while Donovan continues searching.

Lamb uses the loose change to pose as a carol singer to gain entry to the farmhouse. When the guard, Bailey, tries to close the door, Lamb forces his way inside and quickly neutralizes him. He finds a gun on Bailey, unloads it, and climbs the stairs to free Catherine. Lamb tells Catherine that Donovan claimed to want the Grey Books, though Lamb suspects Donovan has ulterior motives. As they prepare to question Bailey about Donovan’s true plan, Ho crashes a double-decker tour bus through the front door.

Douglas emerges from the factory and is immediately tackled by a Black Arrow operative. Duffy takes custody, removes his balaclava, and reassures Douglas that he is from Regent’s Park. He manipulates Douglas into revealing that only four intruders are inside, though both are unaware that Marcus and Shirley are also on the estate and that the hatch cannot be opened from outside. After confirming this, Duffy shoots Douglas with a suppressed pistol, hides the body, and rejoins the assault.