Episode 2

Nick Thomas and Helen Flynn go undercover as a couple, or something close to it, and befriend the wife of a man they are fairly sure is a racist ringleader. The aim is to stop a racewar before it starts. It is the kind of job where the biggest risk is getting comfortable, and the episode keeps that worry close.

Meanwhile, Zoe and Tessa pick up word that a boatload of refugees is due in Dover within days. Two threads, one domestic and one maritime, and the team slowly starts to suspect they are the same thread. The connections between racist ring-leaders, politicians and the killing of immigrants are ugly enough on their own. If the boat is part of the plan, then the whole thing stops being a criminal investigation. It becomes something worse.

But the undercover work gives the hour its best scenes. Nick Thomas and Helen Flynn have to smile across dinner tables at people whose casual conversation would turn most stomachs, and the strain shows in the quiet moments rather than the loud ones. Lisa Faulkner and Tom Goodman-Hill carry that side of it well, with Keeley Hawes's Zoe and Jenny Agutter's Tessa chasing the Dover angle. It's an hour running two kinds of tension at once: the danger of the cover blowing, and the growing suspicion of what the boat's arrival might really mean.

David Wolstencroft wrote it; Bharat Nalluri directed. Peter Firth's Harry Pearce keeps his usual watch, with Esther Hall's Ellie Simm, Heather Cave's Maisie and the rest of the supporting cast filling out the week.

It's a grim subject handled without preaching. The episode trusts the viewer to work out which side of the table the villains are sitting on, and the final minutes leave the covers dangling. That is not an easy trick, and it works.