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.

Half-way through his 12-year prison sentence for an incompetent armed robbery, Jimmy Hands gets a lucky break: he's transferred to a prison from which he can probably escape. He convinces the governor to stage a musical in an old chapel next to the prison's outer wall. He rounds up volunteer actors and puts his escape plan into production. Two other barriers, besides the wall, confront him: the arrival of a nasty inmate, John Toombes, who insists on joining the escape, and Jimmy's feelings of attraction for Anabel, a social worker who agrees to appear in the play. Opening night approaches: is this Jimmy's breakout performance?