Tom is undercover with Customs, but when he goes off to phone in, the rest of the crew are murdered. A Colombian drugs cartel are suspected. Zoe and Tom are sent undercover to 'turn' the girlfriend of the head of the cartel. Tonight also sees the re-appearance of Tessa and she wants revenge on Harry and MI5. Will she endanger Tom and Zoe's life? Will the mole be found before it's too late? Or will Tessa get her revenge and allow someone to die?

The latest round of Middle East peace talks are abruptly halted when a key delegate is killed. With the peace process derailed MI5 starts to investigate a newspaper mogul and renowned Israeli extremist, and a group he's associated with called the November Committee. Can MI5 prevent an assassination attempt on a pro-Palestinian British MP?

Zoe is tried for manslaughter when an MI5 operation results in the murder of an undercover police officer.

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.

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.

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.

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

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.

MI5 has foiled an attempt to murder the Home Secretary, but the plotters use the new emergency powers to imprison Harry. The team must try to frustrate the coup before Harry is executed.

Zaf goes deep undercover to infiltrate an Al-Qaeda cell that is planning an attack on the UK. When Adam learns that the terrorists are on a suicide mission he tries to prevent them from reaching their target, but events soon spiral out of control.

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.

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?

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?