Prologue CONFIDENTIAL From: Metropolitan Police To: MI5 (Lead Development) Subject: Incident 287466 Date: 8 May 2019 1. We are writing to inform you that a 64-year old woman named Willa KARLSSON was admitted to University College Hospital last night in an unconscious state. KARLSSON presents a number of unusual symptoms. For this reason her doctors have been unable to reach any agreement on a diagnosis, but we have been told that one of the possibilities under serious consideration is that she has been the victim of a poisoning. 2. Paramedics were sent to her south London address at 2135 following a call from a downstairs neighbour who reported hearing a loud noise that sounded like a fall. A uniformed police officer who attended the scene observed no signs of violence or forced entry. The neighbour said that KARLSSON lived alone, and described her as quiet, unremarkable and having the dishevelled and careless appearance of ''a bag lady''.

3. In light of the medical assessment, which doctors characterise as ''tentative and rapidly evolving'', our officers have discreetly secured the property and moved residents of the building to a nearby location while experts from Porton Down carry out a thorough examination for traces of poison. Early reports suggest that none has been found, and we note that the paramedics and the police officer who attended the scene last night are all in good health (although they remain subject to close monitoring). 4. An out-of-date identity card found in KARLSSON''s flat indicates that she was until last year an employee of British intelligence. We would like to arrange a meeting with you as a matter of urgency to discuss the possible relevance of this to our investigation. 5. Regards.

CONFIDENTIAL Chapter One Monday, 0900 1 It might come as a disappointment to learn that the natural habitat of the intelligence officer is not the shooting range or the gym mat, the departure lounge of a hot and dusty airport, the safe house or the interrogation cell. It''s not halfway up a ladder aimed at the draughty rear window of a foreign embassy. It''s not even the street, the simple street - narrow, damply cobbled, thick with London fog and Russian menace. No, the natural habitat of the intelligence officer is the meeting room. Spies like to talk. ''You will have heard of a section called Gatekeeping,'' says Charles Remnant. ''In simple terms, we investigate the insider threat - the threat posed by our own members of staff who may have been recruited by hostile foreign powers. What you will not have heard of, however, unless matters have really got out of hand, is the secret cadre of officers we refer to as Gatekeepers.

'' In this case, not just any meeting room, but one at the very top of the building, one at the dead end of a corridor otherwise used to store broken filing cabinets and unused safes. The paint is peeling, the floor stained brown with water from a burst pipe. A sign on the door states ''Electrical Equipment: Strictly No Admittance''. Leonard Flood has worked in the building for seven years and wasn''t aware of its existence until this morning. Dark blue carpet, white walls, two office chairs equipped with the usual array of levers, knobs, switches and even a small hand pump to control air pressure across the lumbar region. He recalls watching the skittery fingers of a new recruit on another floor discover by chance an unexpected button under an armrest, and her panic at the thought she had accidentally triggered a silent alarm or hidden recording device, rather than made an imperceptible adjustment to the angle of her seat. Leonard makes people nervous, despite his best intentions. Even when, as is the case on this particular Monday morning, he is the younger of the two officers in the room by at least twenty years, the more junior by several grades, the one who has been summoned to the meeting not by email or phone, as might have been expected, but by a quiet word in his ear from a security guard as he came through the pods to begin his working day.

''I cannot over-emphasise the sensitivity of what we are about to discuss,'' says Charles Remnant. He smiles tightly to show he appreciates that in this building everyone says such things all the time but then frowns to make clear that on this occasion the words must be taken very seriously indeed. Thirty years clear of the military and he wears his tweed jacket and regimental tie as though the whole damn get-up is unforgivably casual. It is the first time Leonard has been this close to him. The distance Remnant carefully places between himself and his colleagues has created a space where truths and untruths can grow wild: that he has his lunch carried up from the canteen on a silver tray, that he has curated a vast compendium of staff misbehaviour he refers to in private as ''the discipline files'', that he lost his left eye in an accident involving shrapnel, a champagne cork, a swan, a bayonet. At this distance, Leonard thinks, judging from the pattern of scars, the truth is probably more prosaic: that someone once screwed a pint glass into his face. ''The concept is simple,'' Remnant is saying. ''Gatekeepers are officers who carry out covert investigations into fellow members of the intelligence community - into their colleagues and friends, let''s not beat around the bush - to ascertain whether or not they pose a threat to national security.

Is that clear?'' ''Everyone has heard of Gatekeeping,'' says Leonard. ''Everyone accepts that the office has to investigate leaks, misconduct, penetration by hostile agencies. Why is the existence of the Gatekeeper cadre so sensitive?'' ''You''re in this room because people tell me you''re clever. What do you think is the answer?'' The unspoken word ''soldier'' hovers at the end of every question Remnant asks. Leonard turns his face to the window. It is the beginning of a long, hot English summer. Light hums indistinctly but fiercely through the reinforced glass. ''You''re talking about a network of informers who maintain a constant watching brief on those around them,'' says Leonard.

''They spy on the spies, in other words. Which means they must be embedded throughout the office, in every department, carrying out their regular duties in addition to their covert work as Gatekeepers. Your own secret army of accountants, investigators, locksmiths, surveillance-'' ''You''ll understand I can''t possibly confirm-'' ''I don''t know what you expect your Gatekeepers to see,'' says Leonard. ''Anyone carrying out an act of betrayal wouldn''t do it in plain sight. Unless this is an espionage version of the Broken Windows theory. The person who goes on to sell secrets to the Chinese will at some point along the way steal an envelope from the stationery cupboard.'' ''Don''t.