When a Pakistani agent informs them that there is an Al Qaeda cell planning an attack in London, MI-5 go into action. They already have an agent in place in the terrorist cell which is led by Nadif Abdelrashid. When what is supposed to be dry run turns out to be the real thing, MI-5 track four bombers as they walk through the city to what is their final destination. One by one, MI-5 try to take each of them down before the bombs explode but they aren't entirely successful. Lucas North meanwhile has a flashback to being tortured by his Russian jailers and recalls one of the interrogators asking about an operation known as Sugar Horse. For Harry Pearce alarms bells go off as the operation was one of the most guarded secrets and known only to a very few. He is convinced there is a Russian mole in the senior levels of the intelligence community and turns to Bernard Qualtrough, a retired spy catcher, to help him ferret out the turncoat.