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.

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?

MI5 investigates the assassination of a Muslim cleric outside the high court, and uncovers a radicalised Christian who is determined to incite a religious war.

MI5 is put in charge of protecting Niko Grecic, the former right-hand of Tezlia, a Serbian who is put on trial for war crimes. They know that Richard Dempsey, a former MI6 agent who turned on them, is looking for Grecic to kill him before he can testify. Adam, Jo and Zafar must protect Grecic at all costs.

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 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 team steps 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 is 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.

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.

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?