A 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.
Set several years after the end of the Spooks, Harry Pearce is still head of the counter-terrorism department (Section D) at MI5. Harry's team is transporting apprehended terrorist Adem Qasim through London when the convoy is attacked, allowing Qasim to escape and a Central Intelligence Agency operative to be killed. MI5 is humiliated in the press, and relations between British and American intelligence agencies are frayed. Realising that the CIA will demand a scapegoat and that he is soon to be decommissioned from MI5, Harry seemingly commits suicide by jumping off Lambeth Bridge into the Thames.