Warning: This article contains spoilers for the first episode of Slow Horses Season 6.

The Moscow Rules teach agents to watch their backs, while the London Rules advise them to cover their arses. In “Circle of Life,” the first episode of Slow Horses Season 6, Jackson Lamb and his agents discover that they may need to do both at once. Someone is murdering former members of Slough House, while MI5 itself has decided to erase every record proving that the team ever existed.

The season begins by bringing back a face we have not seen since Season 1: Struan Loy, the agent who betrayed his colleagues to the Park and was expelled from the Service. His life has not improved much since then. Still lonely and slightly pathetic, he now teaches martial arts to children while trying to sell small homes built inside shipping containers. Struan believes so strongly in the idea that he lives in one of the prototypes.

That is where he receives Jim and Jane, two people who claim to be interested in the business. They enter, politely listen to his presentation, and accept a drink before revealing the real reason for their visit. They knock Struan down, pour alcohol across the floor, and set fire to the container, blocking the door so that he cannot escape. He burns to death as the assassins walk away with the casual air of people who have simply completed another job.

The death is initially treated as an accident, but the report reaches Emma Flyte, head of the Dogs, who immediately recognises that Jackson Lamb should be informed. Diana Taverner, finally occupying the First Desk position she has always coveted, orders her to do nothing. As far as Diana is concerned, Struan left the Service and does not warrant a special investigation. Emma, however, knows Lamb well enough to understand that even his indifference has limits, particularly when someone who once belonged to Slough House dies under suspicious circumstances.

She finds him at the Italian restaurant near the office and hands over the report. Lamb pretends not to care, because admitting concern would come perilously close to a public display of affection, but he keeps the file and orders Roddy Ho to locate every former Slough House agent. It is Lamb’s way of acknowledging that, despite his betrayal, Struan remained one of his own.

Meanwhile, something is already wrong at Slough House before its agents even know that they are being hunted. An estate agent enters the building and is greeted by Coe smashing a keyboard into his face. The reaction appears excessive until we remember that Coe exists in a permanent state of alert and has noticed a point of light on the wall, which he interprets as a possible laser sight. The visitor is probably only there to assess the property, but his presence raises an uncomfortable question: why is anyone suddenly interested in that particular building?

A much larger problem soon emerges. The salaries have not been deposited, and Roddy discovers that Slough House has vanished entirely from MI5’s systems. It is not simply that the payments have been suspended; the team’s files, employment links and records have all been deleted. Officially, Jackson Lamb and his slow horses no longer exist.

Catherine Standish visits the Park and asks Moira Tregorian to investigate. She learns that this is neither a technical glitch nor an administrative mistake. The order came from the very top. When Lamb hears the news, he announces that he does not work for free and simply walks out, although no one seriously believes he intends to abandon his agents. Their removal from the system has consequences far more dangerous than a missed salary: if they all disappear, MI5 can claim that they were never part of the Service. If they are killed, there may not even be an institution obliged to investigate their deaths.

The episode also begins under the shadow of David Cartwright’s death. River must arrange his grandfather’s funeral and honour one of the greatest figures in British intelligence without being allowed to tell the public what he actually did. To the outside world, David never ran MI5; he was a senior official in the transport department. River attempts to respect the official version in his eulogy, speaking about the difficulties of overseeing so many different forms of transport, while Lamb, sitting in the church, struggles to contain his laughter. Catherine is appalled, but it is difficult not to think that this is exactly the farewell the “Old Bastard” would have expected from Jackson.

At the funeral, Lamb confronts Diana about the disappearance of Slough House from the system. She seems genuinely surprised and insists that it must be a mistake, suggesting that the order may have come from an even more powerful authority or from someone capable of acting in her name. Their conversation also produces an important revelation for Catherine when Diana warns Lamb that she is not Charles Partner and that he will not need to shoot her. Catherine knew that Partner had been a double agent, but not that Lamb had executed her former boss on David Cartwright’s orders.

Diana also speaks to Peter Judd, who mentions the money the Park took from him and claims that it belonged to a wealthy benefactor. The episode does not yet explain how this information connects to the murders or the erasure of Slough House, but it makes clear that Taverner has taken command while carrying agreements and debts that may not be entirely under her control.

Struan, however, is not the only victim. Jim and Jane arrive at the houseboat belonging to Kay White, another former Slough House agent who has never previously appeared in the series. They attack her, throw her body into the water, and use a pole to hold her beneath the surface until she drowns. As they leave, they consult a list and choose their next name: River Cartwright.

River is already vulnerable after his grandfather’s death when he sees Frank Harkness outside the funeral. His father claims that he has come for sentimental reasons, but River does not believe him, particularly after Frank makes a cruel remark about the absence of River’s mother. When River tries to prevent him from leaving, Frank strikes him in the throat. River then jumps onto the bonnet of the car and is eventually thrown to the ground when Frank brakes sharply. Lamb, Coe and Shirley watch without intervening, as though they already know that this confrontation between father and son is only beginning.

“Circle of Life” takes Slow Horses back to its origins by resurrecting Struan and turning the history of Slough House itself into the centre of the threat. Until now, Lamb’s agents have often survived precisely because nobody at the Park considered them important. They were disposable, but that also allowed them to move through the margins of the Service. This time, their invisibility has been turned into a weapon against them: someone has obtained a list of everyone who ever passed through Slough House, begun eliminating them, and ensured that MI5 retains no records proving they existed.

Lamb still does not know who is behind the operation, but he immediately understands the scale of the danger. Struan has been burned alive, Kay has been drowned, River is the next name on the list, and Frank has reappeared at David Cartwright’s funeral. It may all be a coincidence, as Diana insists, but nobody survives that long in espionage by believing in coincidences. The Moscow Rules tell Lamb to watch his back; the London Rules warn him that someone at the Park is already preparing the official version in which the dead will be blamed.