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.

• Peter Firth
• Nicola Walker
• Max Brown
• Lara Pulver
• William Hope
• Alex Lanipekun
• Geoffrey Streatfeild
• Tom Weston-Jones