Episode 5

The team signs up for EERIE, the Extreme Emergency Response Initiative Exercise, a practice run for the day central London gets hit by something nuclear, chemical or biological. It's the sort of drill where everyone is supposed to learn something and nobody is supposed to get hurt. But then the evidence starts to suggest the VX nerve gas attack in Parliament Square is not a simulation. The power cuts out across the Grid, and Harry begins to feel unwell. The drill was supposed to be the worst case; the worst case appears to have moved in early.

From there the episode becomes a test of who trusts the exercise and who trusts their instincts. Tom's leadership is pushed to its limits, and the show makes the most of the ambiguity: every instruction could be part of the drill or the beginning of the real thing. Matthew Macfadyen's Tom has to hold the room together while the ground moves under it. It's the kind of hour where the audience argues with the screen, and that is not a complaint.

Howard Brenton and David Wolstencroft wrote it, Justin Chadwick directed. Keeley Hawes's Zoe, Annabelle Apsion's Bridget Sands, Jo Joyner's Stephanie Mills and the rest of the cast all get their moment in a story that spreads the pressure around. Richard Braine's Phil, Marty Cruickshank's Dot and Alastair Galbraith's John MacLeish round out a cast that sells the drill's terrible efficiency. Peter Firth's Harry, feeling ill at exactly the wrong time, carries the hour's most worrying thread.

It's a clever structure, the training exercise that stops being training, and the episode plays it straight enough that the audience doubts along with the team. The VX threat supplies the stakes, and Harry's symptoms give the hour its chill. It's a quiet kind of horror, and it lands harder for being understated.