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.

Tom and Helen must go undercover and befriend the wife of a suspected hardcore racist in order to prevent a racewar. Meanwhile Zoe and Tessa learn that a boatload of refugees is due to dock at Dover in the next few days. Things worsen when the operatives realise the two things may be connected and their covers may be blown.

Patrick McCann, an IRA leader claims to have information about a nuclear attack on the UK mainland, can MI5 trust the IRA enough to give them what they want in return; 36 hours free of surveillance, free to do whatever they wish?

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.

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?

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...

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.