Witnesses: A Frozen Death, BBC Four review - plummeting temperatures in the Pas de Calais

A thankless task, perhaps, to find oneself following in the footsteps of the berserk Spanish melodrama I Know Who You Are (theartsdesk passim).

Eschewing the bright lights of Paris or the sumptuous sleaze of the Côte d'Azur, Witnesses – or Les Témoins, if you will – is set in northern France, where the skies tend to be grey and the air is cool and damp. There were sweeping shots of lonely beaches and wind-blasted sand dunes, and spine-chilling revelations began to rear their heads when our protagonist Lieutenant Sandra Winckler (Marie Dompnier) accompanied the deeply disturbed Catherine Keemer (Audrey Fleurot, familiar as the unscrupulous lawyer Joséphine Karlsson in Spiral) to the creepy Devil’s Hill, a giant slag heap in Loos.

Somewhat in the manner of such Scandi fodder as The Bridge or The Killing, the story had announced itself with the ghastly discovery of 15 deep-frozen corpses, all male, on a bus parked out in the countryside (pictured right). Although they were all dressed in tidy suits and new shoes, police inquiries soon revealed that all of the deceased had been missing for at least three years. As the facts unravelled, it transpired that all of them had also been on “intimate” terms with Mme Keemer, a photographer who’d walked out on her husband and two young daughters three years earlier. The cops tossed around the notion that Keemer was the killer. Or maybe her husband Oliver (Steve Driesen) had exacted grand guignol vengeance on his wife’s multiple lovers? Winckler wasn’t convinced.

Then Keemer herself turned up in a state of total amnesia, wandering around in a distraught condition muttering “where is he?” before being carted off to hospital. She couldn’t remember her husband or children, or that for five years she’d been renting an apartment in which to entertain the men she contacted via the dating app Fiinder. It even came as a surprise to her to learn that six months earlier she’d given birth.

New outlandish revelations kept cropping up at regular intervals, like the way the mystery killer had used a man called Mauricourt as a front for his purchase of bumper-sized freezers (each big enough for a couple of corpses), then left Mauricourt to starve to death in his own armchair. Keemer’s dazed and confused condition, it seemed, was the result of regular doses of Midazolam, a powerful hypnotic drug which can reduce users to a zombie-like state.

The weirdness of the case was balanced by nuts-and-bolts details of Winckler’s private life. Obviously the idea of a single mother trying to cope with demanding police work while raising a pair of young daughters, not altogether successfully –12-year-old Chloé has already achieved a precocious mastery of how to press mom’s buttons to the most destructive effect – is not wholly original. Nor is the notion of a world-weary detective who drives an elderly but characterful car (in Winckler’s case, it’s a beaten-up sludge-coloured Opel, pictured above).

But Dompnier successfully conveys Winckler’s dogged determination, emotional insecurity, and the awareness that she’s not getting any younger. Her bantering rapport with her fellow-cop Justin (Jan Hammernecker) throws a nicely sardonic light on the policeman’s lot, which is grim and frequently thankless, but can occasionally bring its rewards. There’s going to be  unpleasantness-a-gogo to wade through in the next six episodes, as they close in on a monstrous killer who has seemingly been in the abduction and murder business for decades.

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