Episode 6

The country is about to default on its debts, and the government's answer is to send MI5 to the bank. More than a billion pounds in illegal earnings is sitting in the DeWits Bank, and Section D is asked to get it back before Britain slides into a financial depression. It is the sort of assignment the department is not built for, and the show enjoys watching its people improvise around that.

While the team works the vaults, Ruth's digging into the Basel power meeting keeps paying out, and it throws up a name: Nightingale. Nobody can say yet whether it is a person or something larger, and the episode is content to let the question sit in the background while the money takes the foreground. The two threads pull together by the end, which is the point. Ruth has been after this since Basel, and watching her get closer is half the pleasure.

Dennis Kelly and David Wolstencroft wrote it, and Edward Hall directed. Nicola Walker's Ruth Evershed does the quiet detective work that connects the money to the mystery, while Peter Firth's Harry, Richard Armitage's Lucas, Hermione Norris's Ros and Genevieve O'Reilly's Sarah Caulfield move around her. Ewen Bremner is Ryan Baisley, with Issy van Randwyck as Tina Baisley, Georgia Hall as Leah Baisley and Paul Cawley as the Desk Sergeant.

But a bank job run by spies, with a national economy riding on it, is a good enough hook on its own. The Nightingale thread is the reason the episode feels bigger than its brief, and the reason the season has somewhere to go. The hour gets through the money problem and leaves the bigger question sitting there, which is exactly what a season-long thread should do. It is a rare episode that makes spreadsheet crimes feel like a threat to the nation, and this one does.