S2, E6: 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 the small matter of a President who's about to land in the UK.