Brilliant, Luke thought. We’re racing to track down a team of sociopaths planning to release an apocalyptic bio-weapon on the streets of Britain and I’m saddled with some box-ticking lawyer. But then he reminded himself that if things went wrong, and in his experience they usually did, he’d need the legal top cover provided by someone like Friend. And he had to admit he was still trying to get his head round the different way the Security Service did things from what he was used to on an SIS operation. To Luke, the hierarchy of this investigation seemed laborious, even needlessly bureaucratic. As he returned to his seat and fired up his laptop he ran through the structure in his head. At the top was the authorizing officer, the man who had just delivered them the mission statement and sent them all on their way. If Operation Methuen went belly-up for any reason, the buck stopped with him.

So, who came next? He had to think for a second or two. The controller, that was it. This was the manager who had to make the judgement call on what measures were ‘necessary and proportionate’, a phrase that seemed positively to bounce off the walls everywhere he went inside MI5’s Thames House. The controller, Luke reminded himself, was also the person that he, as an agent runner, was now directly reporting to when meeting and debriefing informants.

But there were yet more layers to this human pyramid, he remembered. On the next rung down were the investigative officers or ‘desk officers’ (such a dreary-sounding name for such a vitally important job, he thought). This was the big team sitting out of sight, upstairs in the situation room, analysing and assessing the material that he and others were bringing in from the sharp end. They must already have been going through the information he’d brought back from Lithuania.

So that left the most important elements of all: the agent, the case, the source. Everyone here at MI5 seemed to refer to them as ‘the CHIS’ – the covert human intelligence source. And this had become his world, his domain, ever since he’d been selected for the MI6 agent-running course down at the Fort. With the ground-floor room in Thames House now half emptied, Luke moved to a seat closer to the front. One of the controllers was addressing the agent runners who remained.