Tense drama series about the different challenges faced by the British Security Service as they work against the clock to safeguard the nation. The title is a popular colloquialism for spies, and the series follows the work of a group of MI5 officers based at the service's Thames House headquarters, in a highly secure suite of offices known as The Grid.
The team has been left shocked by Lucas's betrayal, and with Harry Pearce on gardening leave, Section D has a new leader called Erin Watts. She has brought Calum Reed with her into the team. Harry Pearce manages to win his MI5 employment tribunal and is soon back on the Grid in the thick of the action. Secrets end up being discovered after Max Witt, a retired spy and Harry's colleague in Berlin during the Cold War, is discovered to have been murdered.
Calum is attacked by a gang of street muggers during a routine intelligence-sharing exercise with MI6. The laptop he was carrying is stolen, and contains highly-classified information on MI5's undercover assets. Their names are soon plastered all over the internet, making MI5 look incompetent and putting those top assets' lives in danger.
MI5 receive a red flash: radioactive material has been detected at an airport. They have intelligence that a highly dangerous anarchist with a penchant for dirty bombs has secretly entered the country. They enlist Dimitri to honeytrap the anarchist's only known contact, his sister Natalie. It is a morally tough operation for Dimitri, but through Natalie he gets close to the dirty-bomber. Dimitri's personal connection to them both proves invaluable in the tense and potentially lethal endgame.
Two former inmates are released from jail. One is an undercover MI5 asset and the other is an impressionable wannabe suicide bomber who has been radicalised in prison. They are set to launch an attack somewhere in London, but our team must time it right and step in at the eleventh hour to prevent a massive loss of life.
There is new evidence of CIA agent Jim Coaver's involvement in some recent attacks on MI5. Harry ends up kidnapping his former Cold War colleague but soon ends up realising that he's been looking in the wrong place and Jim could be innocent. Harry, with Ruth's help attempt to steal Coaver's laptop from under the noses of the Americans at the US embassy.
Harry finds himself facing deportation to America following the death of CIA agent Jim Coaver. After some new evidence comes to light about the recent attacks on the United Kingdom and the Government's proposed alliance with Russia, Ruth and her former colleagues in Section D free Harry from CIA custody. Harry's suspicions see him going to another of his former Cold War adversaries.
Harry is still missing, and there is one question on everyone's mind: is he dead, or alive? Section D discovers that Harry thwarted a clandestine operation to smuggle weapons-grade uranium into Iraq to justify the war. Now the rogue officers who masterminded the operation want their uranium back, and they are prepared to go to any lengths to get it. If Harry isn't dead already, is it too late to save him?
The country reaches crisis point when energy supplies are threatened by an explosion at a gas processing plant. With reserves on the verge of running out, Britain is forced to turn to Tazbekstan for help. But in return for energy, the Tazbek Trade and Industry Secretary, Urazov, wants carte blanche to wipe out his enemies on British soil. Is this a price worth paying or can Section D get rid of Urazov in order to secure the deal?
The pressure is on for Section D when a secret meeting of powerful tycoons is stormed by armed terrorists and eight of the world's richest men are taken captive. But this is no normal hostage situation - undercover at the time, Ros is one the hostages. As events spiral out of control, it looks like no one will make it out alive. Is this the end of the road for Ros?
Lucas's past comes back to haunt him when his former torturer Darshavin turns up with intelligence about a bomb in London. Harry fears Darshavin still has a psychological hold over Lucas, but Darshavin won't deal with anyone else. With innocent lives at risk Lucas must confront his demons for the sake of the nation, but can he overcome his nemesis in order to save the day or is the pressure too much to bear?
Harry is puzzled by the sudden death of CIA boss Samuel Walker. Harry is convinced that he did not commit suicide and when another CIA agent dies he begins to think that the deaths have something to do with the power meeting that is being held in Basel. Ros is surprised when her old mentor, Jack Coleville, contacts her and it soon becomes apparent he is involved in what has been going on.
With the country on the verge of bankruptcy, the Government cant pay its national debt payments. Section D are asked to get back more than a billion pounds of illegal earnings from the DeWits bank. The team have to find out what, or who, Nightingale is after Ruth's investigations into the power meeting at Basel pay off.
After a Pakistani intelligence officer is murdered in south London, the head of the Pakistani intelligence service talks to Harry and Ros. They learn that there is a planned attack on some Muslims and there are determined to stop an all out war between the Hindu and Muslim communities.
With India and Pakistan on the brink of war, emergency talks are set up in an exclusive London hotel. Section D faces its biggest challenge yet, as the team tries to avert the first nuclear confrontation since the Second World War. When the Indian delegation abandons the talks, it looks like Nightingale's sinister plan will succeed. Lucas and Ros place themselves in grave danger in a final attempt to defeat Nightingale, but will they make it out of the hotel alive?
The team step in when a British soldier is kidnapped by al-Qaeda. Meanwhile, an old protégé of Harry's returns to the fold after eight years in a Russian prison - but whose side is he really on?
Harry vows revenge by taking down the Head of Russia's intelligence service in Britain with the help of Lucas North. Lucas's loyalties, however, are secretly no longer with MI5 and he exploits Harry's trust to help a Russian submarine destroy Britain's communications system, causing financial and social chaos.
With Lucas back within MI5 he reveals that the Russians interrogated him about Operation Sugar Horse, the most secret ongoing spy operation MI5 ever had. If the Russians know about it, then MI5 has a very high level mole. Harry seeks out his mentor, The Spycatcher, to help him find the mole. Ben is undercover with an Al Qaeda cell; they are planning to bomb London and what looks like a dry run unexpectedly turns live.
The team are stunned to discover that Al Qaeda's number three wants to negotiate. It's a risk, but Harry and the team have to take this chance for peace.
The British economy is on the brink of collapse and if one more bank goes down the entire financial system will implode. Ros goes undercover in the City to flush out a dangerous international financier who wants to bankrupt the country. Suddenly, it is not just the economy, but Ros's life that is at risk.
A teenage boy accidentally stumbles upon a government conspiracy and is instantly absorbed into the world of MI5, ending up on the run. Lucas and the team battle to protect the boy from those who wish to silence him. Harry receives a cryptic message from his asset in Russia which could be the key to bringing down the high-level MI5 mole.
Harry is imprisoned and accused by MI5 of being the Russian mole. Lucas disappears to Moscow in a bid to gather intelligence and get to the bottom of Harry's arrest, whilst the rest of the team try to prove his innocence on the Grid. But with so much evidence against him and a traitor among the team, things aren't looking good for Harry.
With the Russians launching a nuclear strike on London, the only person who can help the team in this desperate hour is the traitor amongst them. With the Russians hunting them down, the team usher their traitor through London in a desperate bid to reach the codes, and the bomb, in time.
Reeling after the explosion at the hotel, Section D are left considering the sacrifices they make as spies. But they are soon back in action, tracking Somalian AQ terrorist Abib on a boat from Tangier. When it becomes clear that the boat is carrying a lethal load, the team fear it is headed to Plymouth for the launch of the newest addition to the Royal Navy's fleet. But is the real target elsewhere, and is it too late to find the UK source of the plan? Lucas's own faith is challenged when he is visited by an unwelcome ghost from his past.
The pressure is on for Beth when she goes undercover for MI5 for the first time. She is tasked with protecting a influential oil baron, Robert Westhouse, when the Team receive intel that an assassination attempt will be made on him at a meeting of top oil dealers. But an unexpected ambush throws things off course, and threatens to dredge up her murky mercenary past. Meanwhile, Lucas visits someone who never expected they would see him again.
Section D are forced to welcome an FSB Officer onto the Grid to help track down Aibek, a rogue nationalist from the ex-Soviet state of Azakstan. Aibek is in the UK to find the last known sample of the nerve agent 'Paroxocybin'. The chase is on, but will the team find a way to stop the nerve agent falling into the wrong hands without ruining Anglo-Russian relations? Meanwhile, Lucas's personal life takes a passionate turn.
When a highly skilled trio of Chinese agents arrive in London for 48 hours only, the team must find out why they are here. Ruth goes undercover in a major scientific research corporation where one Chinese scientist, Jiang, proves to be the ultimate prize. Beth activates an asset from the Chinese Security Services to be Section D's mole on the inside, but has she activated him too soon? Lucas makes a decision that could change his course forever.
The American president is on his way to London to chair top secret peace talks between Israel and Palestine, under the guise of his first state visit. Beth and Dimitri must keep their wits about them as undercover Home Office representatives, tasked with keeping the delegates apart until the talks. When a plot to kill the President is uncovered, they must also track down the assailant. The hotel is put in lockdown as a precaution, but are they locking the threat out? Or in?
Relations between the CIA and MI5 are stretched after an American drone is hacked in Afghanistan whilst in British care, leading to the loss of American lives. Beecher - head of the CIA in London - arrives on the Grid to oversee installation of the US's advanced cybersecurity system, 'Cybershell', to prevent any future disasters. Lucas, out in the field, is tasked with protecting and delivering to the Grid Daniella Ortiz, the young ex-hacker turned White House cryptographer who holds the installation codes. When the Grid is hacked and locked down, the Team are plunged into a nightmare. Will they escape and prevent the theft of all CIA and MI5 intelligence? Will Lucas be able to protect Ortiz when the hackers decide to cut out the middle man?
Lucas is plunged further into the depths of his past when Vaughn takes his manipulations to a new level. Ruth is approached and followed by an imbalanced council snooper who is convinced he's found a dead drop. She initially keeps her distance from him, but when her curiosity gets the better of her she ends up trapped in a situation not even he could have predicted. Lucas and Harry come face to face in the pursuit of the truth.
The team face their toughest challenge ever: tracking down Lucas North, their Section Chief, friend and one of the best agents MI5 has ever had. Harry decides the team are too close, and brings in a specialist to help with the hunt. But it will not be easy to find a man who has nothing left to lose, and as the search becomes increasingly desperate, Harry and Lucas become embroiled in a deadly game of cat and mouse.
Posted undercover at the World Trade Summit for Africa, the team discovers an assassination plot. Initially determined to put their lives on the line to prevent the murder, they are faced with a moral dilemma on discovering the motivate behind the plot.
The government publishes its report on the investigation into death of seven terrorist suspects in a fire at a prison. Harry suspects a cover up, then discovers that one of his team has been implicated.
Adam and Zaf try to find a group of terrorists who have infiltrated Britain. Ros oversees a visit by a delegation of Saudi princes offering a guaranteed oil supply in exchange for nuclear expertise, but the Saudi Trade Centre is attacked by terrorists.
The terrorists start to execute hostages in pursuit of their demand for the release of Al Qaeda prisoners held in Saudi Arabia, but suspicion arises as to the true identity of the terrorists.
MI5 investigate the assassination of a Muslim cleric outside the high court, and uncover a radicalised Christian who is determined to incite a religious war.
An environmental terrorist group is threatening to flood London if the government does not make a secret document public.
Zaf and Adam participate in a mission in Tehran that goes wrong. As the investigation advances, the team realizes they've been lied to as to the goals of their mission.
The country faces a plague outbreak. In a race against time, the officers from Section D have to track down and quarantine everyone who might be infected, while preventing a panic.
A British plane is thought to have been hijacked and taken to an American airstrip. Ros is captured by the Yalta organisation when the team show up and they learn of an international cover-up.
As an Algerian extremist escapes his home detention, the team fear he may be plotting a terror attack on US businesses in the UK with the help of local Iranians. However, working with the Americans to stop the attack proves a trying task given their recently-deceitful behaviour.
MI5 have to intercept a Russian arms dealer who plans to pass blueprints for a nuclear device to Iran. Adam and Ana decide to bring their relationship to a close.
Harry discovers there's a mole at large in the Grid. Meanwhile, Ros and Adam board a plane undercover in an attempt to stop Iran becoming a nuclear power.
A live TV broadcast announcing the new peace accord between the UK, US and Iran is hijacked by armed gunmen. Can Adam and Ros defuse the situation before someone's hurt?
Ros is in grave danger after Bob Hogan discovers she's a traitor. Meanwhile, Harry's brought before Yalta's chief, whose identity shocks him to the core.
The team find themselves suspended from the Grid following what happened involving Ros. Harry learns from an old adversary about a bomb he has planted in London. The team realise that if they respond their lives will be in danger but if they don't it will be public who suffer. A shocking discovery is made when they discover who is behind the latest threat.
Hardliners opposed to the peace between Iran and the West have targeted a London school. Jo is captured by Redbacks, the mercenaries who abducted Zaf.