MI-5 is having a particularly busy day. They have just exchanged spies with the Russians and welcome the return of Lucas North after 7 years of imprisonment. Harry Pierce's Russian counterpart, Arkady Katchimov hints however that they will soon have another agent in custody leading Connie James to try and call off an operation in Moscow that is literally minutes away from happening. Simultaneously, an Al Qaeda splinter group has kidnapped a soldier, Andy Sullivan, who has just returned from a tour of duty in Afghanistan. They release a video threatening to decapitate him if Remembrance Sunday ceremonies aren't canceled. MI-5 learn from the information gathered in Moscow that Al Qaeda is planning a much bigger attack in London. Written by garykmcd

Convinced that his Russian counterpart in London, Arkady Katchimov, is behind the Al Qaeda bombing that killed Adam Carter, Harry Pearce launches an unauthorized investigation. Their task is to find a non-diplomatic source who would provide then with information on Katchimov's activities. Meanwhile, listeners capture an electronic burst leading them to believe there may be an unidentified Russian submarine in British waters. With Ros Myers now the head of the team, they focus on Alexander Beletsky, a Russian who has a relationship with Katchimov. They conclude that the Russians may be looking to tap undersea communications cables and launch a denial of service attack, thereby disabling the Internet throughout Britain. Lucas North is keen to get back to work but Harry insists that Lucas go through the entire re-integration process before he does so. A good decision given that it appears that North may in fact be working for the Russians. Written by garykmcd

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. Written by garykmcd

MI-5 is approached by Mohammed Khordad, the number 3 man in Al Qaeda, for a meeting. Under normal circumstances, they wouldn't even think of meeting the leader of a terrorist group but Harry Pearce is convinced that they should look at any opportunity to reduce the threat of attacks. Khordad is keeping his cards close to his chest and insists that they have to trust one another before they can deal. He tells Harry that a bomb will go off at 3 pm that afternoon unless the Home Secretary publicly absolves two suspected terrorists of any wrongdoing. If he can do that, he will then have more important information for him. As the MI-5 team try find the location of the bomb and Harry tries to convince the Home Secretary of the necessity of making a statement, they also have to deal with another problem: the CIA, which also has a interest in Khordad. Written by garykmcd

MI-5 try to stop a ruthless City financier who is out to force the collapse of a major bank. Manipulating the market is Alexis Maynell's specialty. Ros Myers goes undercover to seek evidence that Meynell is spreading the rumors that the bank in question, Highland Life, is about to go under. The Minister is prepared to freeze Meynell's assets, but only if MI-5 can find concrete evidence that he is involved in insider trading and market manipulation. Ros' life is in danger however when her cover is blown. Connie James finds herself frozen out and under suspicion as Harry continues to investigate the leak around the Sugar Horse network. Harry realizes he may have made a very serious error. Jo Portman's flashbacks are beginning to affect her work. Written by garykmcd

With the military situation in the Middle East rapidly deteriorating and a UN Special Envoy about to hold crisis talks in London, MI-5 come across a teenager selling a highly sophisticated - and classified - electronic disruptor. It turns out the young man saw it being used in the assassination of a photo-journalist and managed to steal it when the assassin was struck by a car. They identify the assassin as one Michael Sands, a former MI-6 agent who had disappeared on an operation two tears ago and who, they assume, has now gone native. As Lucas North babysits the young thief and his mother, the rogue MI-6 agents try to recover something other than the weapon itself. Harry meanwhile receives a coded message from a sleeper agent in Russia indicating that Sugar Horse has been compromised and that she is coming to England. What he doesn't realize is that someone is also trying to frame him. Written by garykmcd

Sugar Horse is revealed to be a network of sleeper agents in Russia. They are there for use only in the direst of circumstances. Just as the West's relations are on the brink - there is much sabre-rattling when the US announces its plans to install its missile shield in former Eastern European countries - two Sugar Horse agents are apparently murdered. All of the evidence points to Harry Pearce who is arrested and is forced to undergo severe questioning. Lucas North travels to Moscow to see what he can learn. A member of the MI-5 team is revealed as a Russian mole and is now desperate to be extracted after killing another member of the team. Written by garykmcd

With the Russian mole inside MI-5 now in custody, the team tries to decipher the information Lucas North obtained in Moscow. It is apparent that the Russian intelligence service had their own equivalent to the British Sugar Horse network and have sleeper agents across Britain and throughout the various security agencies. Information from the mole indicates that a thermonuclear device will be exploded in central London and offers to reveal the identity of all sleeper agents in the UK if they agree to provide protection and a relocation package. The Russians have issued instructions to kill Harry Pearce and all of the members of Section D so getting to the mole's information will be difficult. Harry decides to meet his Russian counterpart to ensure he knows that they will all die if they don't work together to stop the bombing. Written by garykmcd