Episode 6

Relations between the CIA and MI5 are already frayed when this one starts. An American drone was hacked in Afghanistan while under British care, and American lives were lost. So Alton Beecher, head of the CIA in London, arrives on the Grid to oversee the installation of Cybershell, an American cybersecurity system meant to stop any repeat. The Americans want their system installed yesterday, and MI5 has to let them. Colin Salmon plays Beecher with the polite menace of a man who knows he is holding the upper hand.

The job of the day belongs to Lucas North. He has to collect Daniella Ortiz, a young ex-hacker turned White House cryptographer who holds the installation codes, and deliver her safely. It should be a simple run. Then the Grid itself is hacked and locked down, and the building that was supposed to be the safest place in the country becomes a prison with every CIA and MI5 secret inside it. The team has to get out and stop the theft at the same time, and Lucas has to keep Ortiz alive while the hackers decide they no longer need her.

Jonathan Brackley and Sam Vincent wrote it, Julian Holmes directed. Peter Firth's Harry, Richard Armitage's Lucas and Nicola Walker's Ruth carry the main weight, with Shazad Latif's Tariq, Sophia Myles's Beth and Max Brown's Dimitri around them. Simon Russell Beale's Home Secretary, Iain Glen's Vaughan Edwards, Laila Rouass's Maya Lahan and Colin Salmon's Beecher complete the board.

There is a decent thriller buried under the techno-babble, and the locked-down Grid gives the story a claustrophobic push it might otherwise lack. The best moments are the quiet ones between Lucas and Ortiz, two people trapped in a building that is trying to kill them. The fun of the episode is watching the Service fight an enemy that has already got inside the house. It is the rare hour where the danger comes from within, and that is the point the show makes.