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.
- Genre: Drama, Action & Adventure, Crime
- Studio: BBC One
- Keyword: england, spy, mi5
- Cast: Peter Firth, Nicola Walker, Max Brown, Lara Pulver, William Hope, Alex Lanipekun