Episode 10

Angela Wells turns up at Ruth's door with a conviction that will not be argued with. She is an ex-MI5 officer, and she believes the Service killed Princess Diana. She wants Ruth to help prove it. Ruth refuses, and Angela, being the sort of woman who plans her exit before she enters the room, takes matters into her own hands. She arrives at the Grid with a deadly proposition.

The team is suddenly managing two emergencies at once: the woman in the building, and the institution's horror at what she is claiming. The Diana question is the kind of story that never really goes away, and the episode treats it with seriousness while letting the absurdity sit quietly in the corner. A serving officer is being asked to answer for a conspiracy theory that has followed the Service for years. The name still has that effect, even inside Thames House.

Lindsay Duncan is marvellous as Angela, all steel and certainty, and her scenes with the team have the tension of a custody battle over the truth. The Grid, so often the place that interrogates the world, finds itself being interrogated instead, and nobody enjoys the reversal. Angela does. Her certainty is the weapon, and the Grid is not used to being out-believed.

Howard Brenton and David Wolstencroft wrote it, Julian Simpson directed. Peter Firth's Harry, Anna Chancellor's Juliet, Raza Jaffrey's Zaf, Rupert Penry-Jones's Adam, Miranda Raison's Jo Portman, Rory MacGregor's Colin Wells, James Pax's Peter Haigh, John Pirkis's Sir Petrie Meacham and Hugh Simon's Malcolm Wynn-Jones all get pulled into the orbit of Duncan's Angela.

And underneath the whole hour sits a fair question, about how much doubt is healthy and what it feels like when somebody aims it at you. The episode does not answer it. It does not need to.