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.

Harry reawakens sleeper agent Fred Roberts, a renowned chemist and family man. As Fred becomes more deeply involved with deadly terrorists, Tom has a crisis of conscience which could jeopardise his career.

Danny and the team must uncover the truth behind an assassination attempt on a famous novelist.

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?

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...

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

While a deadly hacker turns Britain upside down, Danny decides whether to reveal all to Will, MI5 hunts the source down to an Islamic mosque with the help of MI5's best computer programmers. But things get suspicious when Ruth, close onto the tail of the hacker, goes missing and so does a member of the special team...

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.

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?

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.

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.

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.

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

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.

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.

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 government publishes its report on the investigation into the death of seven terrorist suspects in a fire at a prison. Harry suspects a cover-up, then discovers that one of his team has been implicated.

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.

The terrorists start to execute hostages in pursuit of their demand for the release of Al-Qaeda prisoners held in Saudi Arabia, but suspicion arises as to the true identity of the terrorists.

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.