A car bomb kills the Northern Ireland Secretary and his two daughters. Soon after a British army base is attacked. Bosnian war criminal, Miroslav Gradic, is spotted flying into the UK and the team soon realise that this could be the start of a revenge campaign.

MI5's central computer is being hacked and the number one candidate is school teacher, Gordon Blaney. Zoe goes undercover to get near to him, however she begins to doubt that he is the man they want.