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.

Zafar and Adam race against the clock to uncover the latest bomb. As a new ultimatum is issued and Tash mysteriously disapears, the team is forced to deal with the possibility of a mole within their own organisation.

MI5 suspect a terrorist is planning an attack from his prison cell. Guest stars Jimi Mistry.

The Syrian foreign minister tries to establish subversive contacts with MI5 at the opening of a cultural centre in London. Fiona is desperate to lead this mission and must confront her Syrian past.

Harry upsets the CIA by stopping an extradition to the USA.

A former officer is convinced that MI5 killed Princess Diana and is determined to force the team to prove it.

When a devastating bomb explodes in central London, causing multiple deaths, a terrorist group with a hatred for human life claim responsibility. With their leader facing extradition, the bombers announce that crippling attacks will be launched at ten hour intervals until he is released.

Far-right political party The British Way is threatening democracy so Adam and Fiona go undercover to infiltrate and destroy its campaign.

Adam infiltrates a truck carrying illegal immigrants across Europe in order to stop a would-be terrorist.

A neo-conservative group murder an ex-minister to cover up his past and then they hunt down the journalist who is threatening to blow the story and is under MI5 protection.

An old British traitor is called in to infiltrate a Russian firm suspected of stealing secrets from GCHQ.

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.

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.

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.

Tom Quinn has disappeared after shooting his boss Harry Pearce. He also stands accused of assassinating the Chief of Defence Staff. Chairman of the Joint Intelligence Committee Oliver Mace uses this a pretext to launch a punitive investigation of MI5. Zoe, Danny and the team must prove Tom innocent and save the service from government takeover.

The government introduces Draconian emergency measures in response to a series of terrorist attacks. MI5 fears that influential establishment figures are behind the attacks and may be planning a coup.

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.

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?