A Senior Case Officer and Chief of Section D, Tom joined government agency Mi-5 in 1996 and separated himself quickly from the pack as a result of his cunning instincts, his firm leadership, his ability to excel under pressure, and his trust in the skills of his coworkers. He is unlucky in love, as the dangers of his occupation forces him to keep those he loves at arms length.

Portrayed by: Matthew Macfadyen

A Senior Case Officer, Tessa joined MI-5 in the 1980s and excels best in intelligence-gathering missions. Idolized by teammate Zoe, Tessa nevertheless comes across as cold, calculating, and often combative. In the first series, she was discovered to have been running phantom agents, a secret that Zoe could not keep. Tessa was subsequently fired and started her own private security firm.

Portrayed by: Jenny Agutter

Harry is the head of the counter-terrorism department at Mi-5, having formerly worked his way up through the department as a field officer. Stern and matter-of-fact, Harry has to contend not only with disputes between his employees and his superiors, but also difficult matters of conscience, having to balance what is morally right and was is right for national security.

Portrayed by: Peter Firth

A graduate of Oxford University, Zoe has dedicated her life to her career as a Junior Case Officer at Section D. Skilled in undercover work, she is able to become somebody else with just the right amount of backstory, improvisational instincts, and honest-to-god charm. She currently shares a flat with Danny.

Portrayed by: Keeley Hawes

Young, headstrong , and financially unstable, Danny joined Section D as a Junior Case Officer in 2000. Proficient in the ways of surveillance, Danny is often Tom's first point of contact should a case ever break, despite Harry's worries about Danny's lack of experience.

Portrayed by: David Oyelowo

An administrative officer in Section D and Danny's original flatmate, Helen is still adjusting to her new job at MI-5 when, on the series' second episode, she is asked to replace Zoe on a dangerous undercover mission. Despite showing some grace under pressure during the mission, she is quickly found out by Mi-5's target and is tortured and murdered in front of Tom.

Portrayed by: Lisa Faulkner