At a weekly anti-terrorist meeting, Liz Carlyle, MI5 Intelligence Officer, goes on alert when it is reported that an 'invisible', an unidentified British terrorist, has entered the country. This is the first indicator that something bad may be about to happen. As Liz gets more information and starts to put the pieces of the puzzle together, she realizes that a major terrorist operation is underway. In a race against time, Liz's intuitive skills, her ability to get inside the head of the 'invisible', is the best chance they have to stop them.