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.

Former gang boss turned respected businessman, Richie Beckett pledge money to help rebuild a pier. But Richie's mind is deteriorating, and the other outfit he runs with his sons is under attack by a vicious rival gang.

An illegal immigrant from Albania infiltrates the lives of a group of Londoners with devastating consequences.

Good policing doesn't necessarily mean doing everything by the book. But as the business of crime in London turns to favour the Albanians and Turks, how does a "good" policeman survive?

Brought to the station after nicking some sweets, a waifish Eastern European girl refuses to give even her name. What starts out as a simple case of shoplifting turns into apparent murder, however, after Davies finds the girl's mother strangled. The trail leads to some surprising suspects from a video matchmaking service.