Episode 5

It starts with a stranger on a subway platform. Ruth watches a man kill himself, and the small, horrible act leads her into a series of clues that will change her world. At the same time, the government publishes its official report into the prison fire that killed seven terror suspects, and Harry reads it the way he reads everything: looking for the sentence that is missing. Seven dead in a fire is a tragedy the government wants to file away; Ruth's stranger on the platform is a death nobody has filed yet.

He finds a cover-up, or enough of one to start pulling at it, and the pulling leads somewhere he does not want to go. One of his own team, it emerges, has been implicated in the affair. The episode's two stories run in parallel for a long time before they touch, and the waiting is part of the design.

The setting matters. The subway's anonymity and the prison's containment frame the whole hour, two places where people disappear in plain sight. Gugu Mbatha-Raw's Jenny gives Ruth someone to hold onto, and Tim McInnerny's Oliver Mace adds weight to the proceedings without ever explaining himself.

Zinnie Harris and David Wolstencroft wrote it, Kenneth Glenaan directed. Peter Firth's Harry, Raza Jaffrey's Zaf, Jonathan Barlow's Mik Maudsley, James Dicker's Wesley Carter, Nick Haverson's IT Man, Nicola Goodchild's Prison Security Guard, David Fynn's Policeman at Ruth's House and Gary Milner's Policeman in Cell complete the list.

But the heart of it is Ruth, and Nicola Walker plays her with the quiet alarm of a woman whose world is being rewired around her. The report, the fire, the man on the platform: her week is not getting any easier, and the episode lets her find that out the hard way. It is a strong hour for her, and the show knows it.