The series follows a group of real-life volunteers as they are trained by former spies in espionage techniques, including maintaining a false identity, surveillance, persuasion and recruitment. The programme's psychological challenges, dramatic tension, high production values, and personable cast led to its being called 'the most addictive thing on TV at the moment' by The Daily Telegraph.
Over 5,000 candidates from across the United Kingdom applied to be on the show. The candidates were narrowed down to a pool of eight recruits before the start of the show. As the show progressed, recruits who failed to demonstrate sufficient potential as agents were unceremoniously dismissed from the competition.