The murky world of MI5 is a place of spooks and spies.
It is a secret — sometimes deadly — playground on which the cat and mouse games of the intelligence war are practiced.
In its hidden corners are bugging and surveillance operations, quiet meetings with informants — those termed covert human intelligence sources.
It is about trying to stay one step ahead of your enemy, making them think that you are always watching and listening, that you are always there.
Part of the intelligence war is a mind game. MI5 have to get close. They have many people for many parts.
In one covert operation here they stepped into the sports club of the man they were “babysitting” — trained beside him, travelled with him, while others bugged his phone and placed tracking gadgetry on items within his home.
They get close enough to see and hear everything — but not close enough for their target to know.
In the intelligence war explosives have been removed from dissident republican bombs — and replaced with a lookalike substance.
It is called substitution.
To be able to do it you need information from inside the organisations and from inside their bomb teams. That is how close it gets — and how dangerous it is.
MI5 needs to know who does what — who gives the orders, who makes the bombs, who is in charge of weapons, getting them and hiding them, who is in charge of intelligence-gathering, the money and the training.
Their job is to put the jigsaw together — always trying to see the picture.
The pieces are made out of what they hear, what they see and what they are told.
But there is no such thing as perfect intelligence.
The dissidents have recovered from the arms sting; the bombs at Newry courthouse, Newtownhamilton PSNI base and Palace Barracks in Holywood, which houses the MI5 Northern Ireland headquarters, all evidence of improving technology.
No matter who is arrested, there is always someone else.
That is the way these organisations work — no one is irreplaceable.
As the pendulum swings, MI5 wins one day and the dissidents the next.
That is how these things work. For some the war is not yet over.