Defending their country, deceiving their friends - it's all in a day's work for the agents at MI5, the Security Service. The elite team tackle organised crime, espionage and terrorist activities, as well as the conflicts and power struggles back in the office. There's a car bomb in Liverpool, a siege in the Turkish embassy and the murder of a double agent, not to mention the difficulty of having to lie to loved ones about who you are and what you do.
An abortion doctor is executed by an American pro-life terrorist; Mary Kane. An informant reveals that this is just the tip of the iceberg as Mary has 20 more bombs and plans on using them. To make matters worse she has started training more pro-life groups, within the UK.
Tom and Helen must go undercover and befriend the wife of a suspected hardcore racist in order to prevent a racewar. Meanwhile Zoe and Tessa learn that a boatload of refugees is due to dock at Dover in the next few days. Things worsen when the operatives realise the two things may be connected and their covers may be blown.
Whilst on a routine check of the bugs in the Turkish Embassy, Zoe and all the other people in the embassy are taken hostage by terrorists who wish to negotiate the release of their parents from prison. If Zoe's cover as a PR consultant is blown she will almost certainly be killed. Things worsen when Harry realises that the attack on the embassy was all just a distraction from something much, much bigger.
MI5's best officer, Peter Salter, seems to be joining the enemy. President Bush is scheduled to visit and it seems as though Salter is trying to stop the peace, is this all part of his mission or is he in fact a triple-agent?
Disgraced MP Hampton Wilder has written memoirs with the power to destroy the governement, what's more an old KGB agent is trying to smuggle illegal arms into and out of the country, besmirching the good name of the Prime Minister's right hand man.
Patrick McCann, an IRA leader claims to have information about a nuclear attack on the UK mainland, can MI5 trust the IRA enough to give them what they want in return; 36 hours free of surveillance, free to do whatever they wish?