We had planted an eavesdropping device in it, which should have been live-monitored back at Thames House, MI5 headquarters, but there was an incident going on elsewhere and resources had been diverted.What we didn’t realise was that the target had guessed he was being followed. He and his friends had planned a snatch – to kidnap an MI5 agent. I didn’t know it at the time but they had set up a room, in a house they used, with plastic sheeting on the floor, black flags, video cameras and butcher’s knives – I believe they wanted to try the kidnappee under Sharia law, then behead him online. Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings. Tom Marcus* joined MI5 in his twenties and went on to work in surveillance. Here he talks about his most dangerous missionI’d been in MI5 for a few years when I found myself on the most dangerous mission of my career. I was sent to follow a potential Islamist terrorist in Camden Market, north London, as part of a surveillance operation. He started on foot, then got into a car. I realise now it was there for the snatch. I kept walking, my head down, but suddenly, as I got closer, the van pulled away and raced off. I still don’t know what spooked them but it had gone down to the wire.Later, when another group of officers searched the address and found the knives, flags and camera, I realised how close my brush with death had been. I joined MI5 in my 20s – I had previously been in the Royal Engineers and was noticed by one of the colonels. I was very fit and had impressed him by standing up to him, so he sent me to special operations selections, which led to me joining the service. I followed the target on foot and in the car all day, then that evening he parked in a very rough part of London and walked through a narrow alley towards a mosque for last prayers. I followed close behind. As I walked through the alleyway, I realised my radio wasn’t working properly. It was raining heavily and visibility was poor. The only thing I could see was a pair of tail lights at the end of the alley – a Transit van was hovering there. I went through several traumatic events and my nightmares became unbearable Armed British police walk in the check in area of Gatwick Airport, SussexCredit:CARL DE SOUZA/Getty Images Despite my troubles, I’d recommend it as a career. It’s refreshing to see how much the service has changed. They used only to recruit Oxbridge graduates with brains the size of planets, but now they appoint people from all backgrounds who won’t stand out if they’re sent to a pub or a council estate. Ultimately, to be a spy you need loyalty, integrity and to be a team player – qualities that, deep down, most people have in them.*Name changed Soldier Spy (Penguin Books Ltd, £20) by Tom Marcus, is available for £16.99 plus £1.99 p&p from The Telegraph Bookshop (0844 871-1514, books.telegraph.co.uk) I began working in counterterrorism in Northern Ireland, and met my wife, who was working in the same covert unit. She matched me glass for glass at a Christmas party and we got married four months later. Our colleagues called us Mr and Mrs Smith.During my eight years in the service, I specialised in surveillance and adopted all sorts of disguises, from a homeless person to a painter. I also played a part in foiling al-Qaeda’s plot to bomb Manchester landmarks over the Easter bank holiday in 2009.But I also went through several traumatic events and my nightmares became unbearable – I was diagnosed with post-traumatic stress disorder and had to leave MI5.It was hard to find work because I couldn’t disclose where I’d been working for the past few years, but after some time, I’ve finally settled.