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 can't pay its national debt payments. Section D is asked to get back more than a billion pounds of illegal earnings from the DeWits Bank. The team has 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?

Reeling after the explosion at the hotel, Section D is 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 fears 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 an influential oil baron, Robert Westhouse, when the team receives 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 is 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 Daniella Ortiz to the Grid, the young ex-hacker turned White House cryptographer who holds the installation codes. When the Grid is hacked and locked down, the team is 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 faces 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.

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.