Episode 3

Somebody gets into MI5's central computer, and suddenly every agent's name is a weapon pointed at the Service. The hack strands operatives in danger zones across the world, with no way to know who is compromised and who is safe. The prime suspect is a school teacher with links to left-wing extremists, which is either a gift from above or a very neat piece of misdirection. The first thing the team loses is certainty, and the episode makes that loss feel physical. Agents who spent years building covers have to wonder if the covers are already gone. It is the kind of threat you can't shoot, and the episode knows it.

Zoe goes undercover to get close to him and find out what he knows. It's the show's oldest trick, the undercover approach, and it still works because the danger is specific: if the breach goes all the way, every cover in the building is dead. Matthew Graham and David Wolstencroft wrote the script, Rob Bailey directed, and the hour moves at the pace of a very bad week. The episode trusts the audience to keep up, and the pacing rewards attention.

Peter Firth's Harry runs the damage control, Keeley Hawes's Zoe carries the undercover scenes, and Christopher Fairbank's Gordon Blaney and William Gaminara's Victor Gleeson give the hour its suspect list. Around them the smaller roles do their work: an IT Project Manager, a school teacher, a young woman called Daniella. The episode keeps its cast tight and uses everyone.

But the episode's best trick is making a keyboard feel like a weapon. The action is quiet, and the horror comes from not knowing who else knows. For a show so often built on chase sequences, it's a reminder that the threats that arrive by modem were always the scariest ones.