Episode 7

Some threats are loud. This one is quiet and digital and everywhere at once. A group starts hacking into systems across the country: tampering with the ingredients in painkillers so that everyday drugs turn lethal, wiping bank balances, scrambling traffic lights. Britain's infrastructure stops behaving, and nobody can see the hands doing it. The scale is what unnerves: it isn't one target, it's the country.

MI5 brings in its best programmers and follows the trail to an Islamic mosque. The mosque is where the trail ends, but the episode is careful about what that means and what it doesn't. Danny, meanwhile, has a decision of his own to make about how much to tell Will. And Danny's choice about Will hangs over everything, a smaller version of the same question, and just as hard to answer. Then the professional and the personal collide. Ruth goes missing while close on the hacker's tail, and a member of the team disappears too.

The episode is populated by the ordinary people whose world has stopped working. James Barriscale's John, Kate Benedict's headmistress, Clare Corbett's teacher, Roderic Culver's pharmaceutical executive, Waleed Elgadi's Arab at Computer and Joey Jeetun's computer technician all move through the story as the damage spreads. Kay Burley turns up as a Sky newsreader, which feels right for an hour about a country watching itself come apart. Peter Firth is Harry, Richard Harrington is Will. Raymond Khoury and David Wolstencroft wrote it; Bill Anderson directed.

The episode is about how quickly modern life comes apart when the systems underneath are turned against us. It doesn't need to spell that out, and mostly it doesn't. The vanished agents give the second half its engine, and the episode keeps their fate close to its chest. It's the closest the hour gets to a personal stake, and it works. The ending is deliberately unsettled, which suits an attack that never announces itself.