When the President of the United States makes an unscheduled trip to the UK, it's a huge and complex operation. MI5 must work with the CIA and the President's security team to ensure his safety throughout, but this becomes a battle for power and as each new threat comes in, it has to be dealt with. Tom and Christine are thrown together and there is very obviously more to their relationship than just work, but there's also the small matter of a President who's about to land in the UK.

Britain is in the midst of a 'winter of discontent' and MI5 have been closely monitoring the re-emergence of union strike action. Firefighters, nurses, teachers, air traffic controllers, council workers and rail unions have all clashed with New Labour and their strike action is bringing the country to a standstill. Unions dominating the political agenda poses a real threat to the stability of the country, but MI5 have had word of something far more serious - they've been informed that a section of the British Army is planning strike action of its own.