THEY STARTED repeating Canterbury Tales on BBC One last night, surrounded by the populist trivia commissioned in the Greg Dyke era. Now there’s a funny thing. They decide to show one of the BBC’s few critically acclaimed, original dramas of the last few years just as Channel 4 is screening Sex Traffic.

Sex Traffic is the kind of harrowing, deeply researched, carefully crafted, socially committed and angry dramas that is supposed to have characterised BBC output in its golden age — a Cathy Come Home for the Noughties. It’s the sort of production that gives public service broadcasting a good name and prompts the question: “Why doesn’t the BBC make stuff like this any more?” Especially if it’s opposite Alan Titchmarsh or Trinny and Susannah.

But it’s probably just a coincidence. Anyway, I hope Jimmy Nesbitt in The Miller’s Tale didn’t pull too many viewers away from Sex Traffic, because it was blistering. It took one of those “issues” about which you may have felt you had a vague idea, and showed you that it is, in fact, much, much worse than you could have imagined.

I suppose if you were an ostrich living in a Trappist monastery on a desert island you could have missed the news that the sex trade in Western Europe is now staffed by poor Eastern European girls, many of whom have been duped into it, and are run by vicious pimps. If it achieves nothing else, Sex Traffic will have done us all a favour by showing what this really means.

At the core is the story of Elena and Vara, naive young sisters from Moldova, tricked into leaving for London by Vara’s boyfriend. Actually he is throwing them down a human rubbish chute, their misery and humiliation accelerating as they drop. He takes their passports, then sells the girls on to the first of a succession of middlemen, who take them not to London as promised, but to a brothel in Sarajevo. Each dealer takes his cut. The girls are “broken in”, beaten, raped, abused and threatened. Elena has left her baby son at home with her mother. The pimps tell her they will have him killed if she crosses them.

That, though, is the straightforward bit. The writer Abi Morgan says that she can back up every incident in the girls’ story with personal statements. What will make Sex Traffic more controversial, and open to accusations of dramatic exaggeration, is the parallel plot. Agents employed by Kernwell, a private American security firm operating in Bosnia as part of an international police force, are trafficking women themselves.

The story opens with a love-struck young Canadian Kernwell agent trying to “buy” his own girlfriend when he discovers that she is to be sold abroad. He is caught, and a massive cover-up ensues, complete with cynical corporate lawyers and ruthless chairs of boards. Meanwhile his girlfriend is thrown off the tiny boat taking her from Albania to Italy when an Italian police launch appears. This is standard practice, we learn. The police must try to rescue her, allowing the traffickers to escape with the rest of their cargo.

Morgan is less willing to be specific about the inspiration of the Kernwell plot, suggesting that inquirers explore the internet. Unlike the story of the Moldovan girls, it feels more like informed speculation than dramatised fact. Some might see it as yet more British liberal Yank-bashing. It would be a pity if it undermined the force of the drama as a whole.

Destined to bind the two plots is John Simm as Daniel Appleton, an earnest but mousy translator for a British refugee charity, who morphs into an investigative journalist when he is sent to Bosnia to learn about the trafficking routes. It is a surprisingly low-key role for Simm. But the drama is about characters gradually realising the full horror of what is going on: the sisters, the Canadian, and the Kernwell chief executive’s wife. Simm’s character fulfils that role for the audience.

In the end what is most shocking is the casual brutality of the traffickers. We start with a girl thrown overboard. Later Elena manages to get a message to her mother in Moldova. When her black leather-clad Serb pimp finds out, he strides into the girls’ shared bedroom and shoots another girl dead in front of them. She has sores on her face and is no longer marketable, but he is making a point. So is Abi Morgan, and more powerfully than a hundred documentaries could.

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