An abortion doctor is executed by an American pro-life terrorist; Mary Kane. An informant reveals that this is just the tip of the iceberg as Mary has 20 more bombs and plans on using them. To make matters worse she has started training more pro-life groups, within the UK.

Whilst on a routine check of the bugs in the Turkish Embassy, Zoe and all the other people in the embassy are taken hostage by terrorists who wish to negotiate the release of their parents from prison. If Zoe's cover as a PR consultant is blown she will almost certainly be killed. Things worsen when Harry realises that the attack on the embassy was all just a distraction from something much, much bigger.

The baby of a celebrity couple is kidnapped and they call in MI5. Fiona goes in undercover to gather more information about the couple against Adam's wishes.

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.

Two former inmates are released from jail. One is an undercover MI5 asset and the other is an impressionable wannabe suicide bomber who has been radicalised in prison. They are set to launch an attack somewhere in London, but our team must time it right and step in at the eleventh hour to prevent a massive loss of life.

MI5's best officer, Peter Salter, seems to be joining the enemy. President Bush is scheduled to visit and it seems as though Salter is trying to stop the peace, is this all part of his mission or is he in fact a triple-agent?

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?

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

Adam and Zaf try to find a group of terrorists who have infiltrated Britain. Ros oversees a visit by a delegation of Saudi princes offering a guaranteed oil supply in exchange for nuclear expertise, but the Saudi Trade Centre is attacked by terrorists.

Disgraced MP Hampton Wilder has written memoirs with the power to destroy the government, what's more an old KGB agent is trying to smuggle illegal arms into and out of the country, besmirching the good name of the Prime Minister's right hand man.

After stealing a large amount of money from a bank, a man is found crucified on the moors. The Treasury call in the team to investigate, but when Harry decides to take the mission further Danny's life is put in danger.

The president of the USA makes an unexpected stop in London. Tom and his team are forced to work with the CIA. A power struggle between the two secret organizations begins, but will they stop fighting before a terrible mistake is made? And will Tom ex-girlfriend leave well alone or will she have to be taught a lesson?

A French scientist has gone renegade and taken a top secret weapon called Firestorm with him, which he is threatening to sell to the highest bidder. Now he has ended up in the UK, and MI5 is forced to work with French Intelligence. But the British want the technology. They have to steal the technology without the French finding out, using an agent that if caught has no affiliation to Five. But there is a burglary at Harry's flat which puts the operation in jeopardy. Or does it?

Saturday morning. Danny and Colin practice their firing technique on MI5's gun range. Zoe and her fiancé Will are still madly in love. Ruth watches old movies at home whilst longing for real-life romance. A North Sea mission places additional stain on all relationships...

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.

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?

When known mercenary Robert Morgan is spotted at a top-secret disused Soviet arms dump, he obviously has to be brought in for questioning. Is it all right to blackmail innocent people to get the information you want? Is it acceptable to threaten the lives of a suspect's family?

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

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.

Posted undercover at the World Trade Summit for Africa, the team discovers an assassination plot. Initially determined to put their lives on the line to prevent the murder, they are faced with a moral dilemma on discovering the motive behind the plot.

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?

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 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.