As I sat at an arranged meeting point in a public car park I had the unnerving feeling I was being watched.
A middle-aged man smoking a cigarette walked slowly by, then stopped momentarily. I glanced at him suspiciously through the rear view mirror, certain he was spying on me.
Just then, precisely on time, a shiny black car with blacked out windows pulled up. Two casually dressed men in the front nodded a silent greeting and a door opened for me to climb in.
There were no introductions or names exchanged during the short drive to our destination - the Northern Ireland headquarters of MI5, based within Palace Barracks in Holywood.
It is inside this building that real life 'spooks' are working to foil terrorist atrocities.
This is where they seek to glean intelligence on dissident republican terrorists, where informants are identified for recruitment and spy operations are launched.
While television series such as Spooks have glamourised life in the secret services, I was soon to discover that, on the surface, the daily reality can be much less dramatic.
As we approached the entrance of Palace Barracks armed guards in black bulletproof jackets thoroughly checked my escorts' security clearance.
Two attempts have been made by dissident republicans to bomb the base in recent years. A car bomb exploded at the main gates of the barracks in 2010 and last year a device detonated inside a postal van at the base.
We drove through two security gates and onto the Army camp, where we followed a winding road past well-kept MoD accommodation, neat rows of Army houses and a group of soldiers.
The car came to a stop at the front of a small concrete building and I was asked to leave my phone and any recording devices in the vehicle.
Inside the clinically clean entrance hall an armed guard was waiting with a security pass and a digital code.
On the wall beside the door a digital sign flashed a warning in red letters that the security threat in Northern Ireland remained at "severe".
Beyond glass doors to my left were airport style X-ray machines, leading into a green carpeted, Portacabin-type room that could be divided in two by a white, sliding wall.
The only furniture was a computer, a desk, half-a-dozen green leather chairs, a small round coffee table and a wall clock.
This claustrophobic room is where last stage interviews are conducted for hopeful MI5 recruits who have already passed several tough assessments.
Surprisingly, we bypassed the X-ray machines. Nobody checked my handbag or searched me. I questioned this and was told: "We know you're not going to pull out a knife." This confirmed my suspicions that I had been thoroughly vetted well in advance of this visit.
We stepped outside and there, towering ahead of us, at the top of a flight of steps was the very building from where the fight against Northern Ireland-related terrorism is directed. Behind several storeys of red brick wall and green-tinged bullet and blast-proof windows lay the shadowy world of secrets and subterfuge.
The headquarters were built in 2007 when MI5 took the lead role for intelligence on Northern Ireland-related terrorism.
As the glass door was held open I stepped inside and became the first journalist to enter the building with permission to report from inside.
A row of bulletproof glass pods divided the entrance hall in two. The main part of the building cannot be accessed without going through one of the cylinder portals, which can only hold one person at a time.
Security passes had to be scanned and a code typed into a digital pad before the door of the pod opened.
I stepped inside and the pod door slid closed, holding me prisoner for a few seconds until another door opened and I could exit.
The portals were designed to contain an intruder in the unlikely event of an unauthorised entry. With the sweep of a security pass another large, sturdy door was opened and we stepped into the heart of the building - a startlingly bright and spacious atrium several storeys high.
Light bounced off glass panels, introducing daylight into the physical centre of an organisation that has been perceived for many years as "shadowy" and "dark".
A large, glass-panelled staircase rose from the centre of the atrium to the top level of the building.
The names of numerous departments I was never aware existed were signposted on a double lift.
We stayed on the ground floor, where one corner was furnished with green leather sofas, coffee tables and a newspaper stand. Nobody was there.
Alongside it was an empty canteen, with the counter shutters closed.
Behind a glass wall on the opposite side was a room called the Learning Centre, that looked like a small library, with books and manuscripts stacked neatly on bookshelves. It, too, was empty.
Apart from my escorts, the only other people I could see were two smartly dressed women in quiet discussion outside a closed room. I was told they were from one of the agency's many legal departments.
Everything was symmetrical and orderly. It was also strangely quiet. It had the feel of a modern lounge in an empty airport.
All doors within my vision were firmly shut, protecting the secrets and identities of the men and women commonly referred to as "the spooks".
While I had not known what to expect from the visit I was quite surprised and perhaps a little disappointed, at how normal everything seemed.
An intelligence officer I met during my visit, a woman in her 30s called Mel, described working for the agency as "not much different to any other job".
She handles agents - or 'informants' - for a living. The information she secures has helped to save many lives.
"It really isn't that different to any other job," she said.
"We have normal working hours and we don't take our work home."
Somehow I doubt it, although she did sound convincing. I was acutely aware that behind those shut doors the real action was taking place - the perceived boredom of the building belying its life and death importance.
Inside those closed rooms is where major stings were planned against terrorists like Terence and Gavin Coney, Sharon Rafferty and Sean Kelly, who were jailed in connection with a dissident republican training camp on the outskirts of Omagh.
Intelligence officers in those rooms would have spent months planning secret surveillance operations, the planting of listening devices and analysing information secured from their informants on the ground.
It would have been behind those doors that a covert operation was launched to foil an alleged dissident republican plot to target judges and police officers in Northern Ireland.
Seven men were arrested in 2014 after MI5 placed listening devices in a house in Newry as part of that operation.
By its nature, MI5 will always attract criticism because of a perceived lack of transparency and accountability.
The agency is keen to dispel this notion, while still maintaining the integrity of its processes.
"Misperceptions and misrepresentations of the past mean that trust is lower in Northern Ireland than in the rest of the UK," MI5 director general Andrew Parker said in a statement.
That is why it opened its doors to the Belfast Telegraph.
Mr Parker said: "We will be able to do our jobs better if we have the support of the whole community.
"So - while there are sometimes limits to what we can say about how we stop attacks, in order that we can keep doing it - I want people in Northern Ireland to see that we are a modern, inclusive organisation operating to the highest ethical standards within a clear legal framework, and subject to proper independent scrutiny."
As I left the 'ghost' base my transport and two escorts were waiting.
We travelled back to our earlier meeting point with barely a word spoken.
As we drove away in different directions, I still couldn't shake the feeling that I was being watched.
Was I? I guess I'll never know.
Tomorrow: Which terror groups pose the biggest threat and an MI5 spy reveals how she recruits informants