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

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.

An environmental terrorist group is threatening to flood London if the government does not make a secret document public.

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.

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?

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.

Lucas's past comes back to haunt him when his former torturer Darshavin turns up with intelligence about a bomb in London. Harry fears Darshavin still has a psychological hold over Lucas, but Darshavin won't deal with anyone else.

With innocent lives at risk Lucas must confront his demons for the sake of the nation, but can he overcome his nemesis in order to save the day or is the pressure too much to bear?

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.