Tense drama series about the different challenges faced by the British Security Service as they work against the clock to safeguard the nation. The title is a popular colloquialism for spies, and the series follows the work of a group of MI5 officers based at the service's Thames House headquarters, in a highly secure suite of offices known as The Grid.

When Martin Luxford (Hugo Speer) leaves jail, he decides to form a swing band, having been taught to play the saxophone by his cellmate Jack. Returning to his native Liverpool, Martin pulls together a backing band of misfits and loners, and recruits his ex-girlfriend Joan (Lisa Stansfield) as a singer. Things are complicated somewhat by the fact that Joan is now married to the policeman who arrested Martin, and when the band's first gig - at a heavy metal pub - goes badly, it seems as though the road to musical success may be a rocky one.

The wise-cracking Fitz is a brilliant but flawed criminal psychologist with a remarkable insight into the criminal mind.