Episode 6

Two crises at once, because one is never enough. Adam Carter and Ros Myers board a plane undercover to identify a courier carrying nuclear weapons technology to Iran, while Jo Portman and Malcolm Wynn-Jones race to prevent the poisoning of London's water supply. Iran has threatened the poisoning if the courier does not reach his destination, so the two jobs are really one job with a countdown attached. Adam and Ros have to work a cabin full of strangers without blowing their cover, while Jo and Malcolm chase a threat that could kill a city from the inside.

Harry Pearce, meanwhile, is coming to a worse conclusion. There is a mole at large in the Grid, and he has to hunt inside his own house while the rest of the team works the case. The episode keeps all three plates spinning, which is a lot of plates, and for the most part it manages, because each thread is simple enough to follow even when they interlock. The team is stretched thin, and the episode is happy to show them running out of people.

George Tiffin and David Wolstencroft wrote it, Brendan Maher directed. Rupert Penry-Jones and Hermione Norris do the undercover work as Adam and Ros, with Miranda Raison's Jo Portman and Hugh Simon's Malcolm Wynn-Jones on the water. Peter Firth's Harry Pearce, Gemma Jones's Connie James, Matthew Marsh's Bob Hogan, Sam Graham's Richard Rolfe, James Laurenson's Sholto and Tom Binns's search officer fill out the hour.

The mole thread is the one to watch. Everyone else is on a plane or in a treatment works, and the person doing the damage is probably sitting in the office next door, watching them all leave. And the hour knows exactly how much unease that image is worth.

It is a busy hour that earns its busyness. Everything connects, and the connections are the point. Nobody gets to be an island, and that is the message of the season so far. The water job is the scary one, precisely because it is so ordinary.