Episode 4

Julia Montague lies in intensive care and two people are dead. The blast that put her there has to be explained, and the explanation is already turning toward the man whose job it was to stop it. David Budd is now a suspect in his own protection case.

Mike Travis, appointed Acting Home Secretary, hands the investigation back to Counter Terrorism Command. The police think Tahir Mahmood carried the bomb, but the CCTV does not quite prove the explosion came from the briefcase. And Budd is the one who searched that case and waved Mahmood through, which is why Sharma and Rayburn are going through his flat. The next day the Prime Minister announces that Montague is dead. Commons footage shows adviser Rob MacDonald handing Mahmood the case, and Travis tells MacDonald to stick to their agreed story before anyone can interview him.

Budd's reaction to all of it is a suicide attempt that fails because someone has already replaced the bullets in his hidden pistol with blanks. That detail sits in the episode like a threat: somebody out there is managing him. He goes back to Montague's hotel and finds the security footage has been doctored to remove Longcross's visit. Then, on Commander Sampson's orders, he sits in on Rayburn's interview with Nadia Ali, the suicide bomber from the train, and watches her fail to pick out the bomb-maker from the pictures, Mahmood's face included.

Jed Mercurio wrote it; John Strickland took over the directing. Gina McKee's Sampson and Nina Toussaint-White's Rayburn get the work of the week, with Ash Tandon's Sharma, Sophie Rundle's Vicky and a wall of real news presenters, Laura Kuenssberg and Mishal Husain among them, keeping the world outside the investigation moving.

It is a grim hour, and the show trusts the audience to feel the walls closing in without being told to. Budd is a long way from the hero of the train now.