A fellow agent informs Tom that young boys and men from a mosque in Birmingham are being trained as suicide bombers. They need an undercover spook, but who should or can go?

Tom ignores Harry's demands to end his relationship with CIA officer Christine Dale. Christine hands Tom a secure communication which she has accidentally received from high up in the CIA. It says that an American assassin has come to the UK for a hit and that the CIA are going to take him out, without informing Five or any of the security services. Tom can't tell Harry what he knows so enlists Danny and Zoe to help him. However, things start to go wrong and tonight's episode is all 'Smoke and Mirrors'. Will the truth be uncovered before something goes terribly wrong? Has Tom gone bad?

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 shows up and they learn of an international cover-up.

As an Algerian extremist escapes his home detention, the team fears 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.

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 finds 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 realizes that if they respond their lives will be in danger but if they don't it will be the public who suffers. 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.

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.

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.

Tom's skills as leader are tested to their extremes when they take part in an Extreme Emergency Response Initiative Exercise (EERIE) to test their response to nuclear, chemical or biological act in central London, but when the power in the grid is cut and Harry starts to feel ill, they soon begin to suspect it's for real.

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.

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 tries 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 Tom's family trapped in his house with a bomb, and nothing Tom or the other spooks can do but watch on. There is a huge explosion and the credits begin. However, there is no time to focus on these events as a Serbian warlord has been seen entering the UK vowing revenge for the death of his sons in a NATO bombing raid. Zoe is sent undercover to track him down, and stop any more deaths.

MI5's mainframe has been hacked into. But Tom and his team think they have found a lead in the form of an I.T. teacher, so Zoe is sent undercover to get close to him and find out his secrets. Will they be able to stop the culprit before the inner sanctum is breached and every agents' identity revealed?

A new series of strikes prompted by the unions has the team worried. And when news of a 'strike' by the army arise, Tom is sent undercover to investigate. Meanwhile Zoe takes charge back at the Grid, and is tasked with moving spent uranium cells across the country after rail strikes prevent it taking it's usual route. Will it get to Dover on time? Or will it fall into the wrong hands?

It's Fiona's birthday, and she's not going to forget it! While inspecting a possible Islamic terrorist safe house, they are kidnapped. With the threat of his son's, his wife's and Danny's life, Adam must choose who won't live to see another day, Fiona or Danny? The prime minister's life is in danger. Will MI5 give into the kidnappers demands? But then to everybody's surprise... Danny makes the ultimate sacrifice.

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 ushers their traitor through London in a desperate bid to reach the codes, and the bomb, in time.