Episode 8

Britain is on strike, the trains are off, and Whitehall is working round it as best it can. Into that mess comes a worse rumour: a decorated Army Major, Sam Curtis, is suspected of plotting a mutiny. The mutiny rumour is the kind of story that gets laughed off in peacetime, and nobody here is laughing. Tom goes undercover to find out, and what he finds is the plan for a dirty bomb, built from stolen radioactive waste, with the strikes already providing the chaos it needs. Tom's undercover work means living with the men he is investigating, and every handshake is a risk.

Meanwhile Zoe is holding the Grid together, trying to move spent uranium cells across a country whose railways have stopped. Dover is the deadline. But the question of whether the shipment reaches Dover, or falls into the wrong hands first, keeps the hour ticking. Zoe's week is a logistics nightmare with a national-security price tag, and she takes it on without a fuss. It's two operations running in parallel, one quiet and one logistical, and the episode cuts between them with confidence.

Steve Bailie and David Wolstencroft wrote it, Ciaran Donnelly directed. Reece Dinsdale's Major Curtis is a properly unsettling antagonist, all discipline and delusion, and Leo Bill's Corporal Eric Woods and Kieran Bew's Bryant do good work around him. Peter Firth's Harry, Keeley Hawes's Zoe, Matthew Macfadyen's Tom, Megan Dodds's Christine and Shauna Macdonald's Sam Buxton give the hour its familiar, dependable centre, with Michael Hodgson's Derek Hanson and Roger Brierley's Mr. Getty in support.

The strikes make a clever backdrop: ordinary life grinding to a halt while the people paid to see the bigger picture work through the noise. It's the kind of episode that makes a simple word like shipment sound like a countdown. The show is at its most quietly efficient here, and that efficiency is the point.