The Sixth Commandment review: What starts with so much promise descends into tawdry true crime

Do you know your Ten Commandments? The sixth, which forms the title of a new four-part BBC One true-crime serial, is perhaps the most famous. “It rhymes,” a creepy, disembodied voice tells us, “with ‘thou shalt not thrill’…” Murder, then, is the sin at the heart of The Sixth Commandment, the latest drama from Sarah Phelps, best known for her Agatha Christie adaptations. This, however, is not a whodunnit, but a whydunnit.

Timothy Spall is Peter Farquhar, a newly retired schoolmaster who is turning his hand to the occasional university lecture. It is there that he meets Ben Field (Éanna Hardwicke), a student who seems unusually attentive to Peter’s needs – Peter, as Ben rapidly discovers, has struggled his entire life with the project of reconciling his Christian faith with his homosexuality. “I have to be untouched, unloved, live only a fraction of my life,” he tells his brother Ian (Adrian Rawlins) when confronted about the way that Ben has inveigled into the Farquhar nest. “Ben has brought me to life.”

But what are Ben’s intentions? Despite being a young man living with a girlfriend, he promptly breaks up with her and moves in with Peter – along with his friend Martyn (Conor MacNeill) – and soon enough proposes to become “betrothed”. “I don’t want sex,” Peter tells Ben. “I want to hold and be held.” Within weeks, though, the older man is experiencing extreme dizzy spells, which Ben, strangely, attributes to an undiscovered issue with drinking. And so The Sixth Commandment switches gears, turning from an intimate character study – a rare portrait of later-life isolation – into a chilling examination of a devious gaslighter and, ultimately, murderer.

There are two victims in this story: Peter Farquhar and Ann Moore-Martin (Anne Reid), both of whom were seduced and scammed by Ben Field between 2015 and 2017. The show is notionally dedicated to the pair – it carries an epigraph at the start – but the intimacy is at times uncomfortable. Both Peter and Ann have been single their entire lives (Peter was 69 at his death, Ann 83 at hers) and their latent sexuality was exploited by Ben. Peter’s story, particularly, of the repressed homosexual teacher, has been often told but rarely so sympathetically, but Spall’s excellent performance is rapidly superseded by the figure of his killer.

From the moment he appears on screen, Ben is a malevolent presence. Whether by design or coincidence, Hardwicke is unsettling. Entirely devoid of charisma, he stalks through his scenes: there is no sense of interiority, scarcely a hint of motive. It is, the show contests, a form of evil. In the second half of its run, the focus turns from the victims to their families and the police investigation. The writing, while always sharp, becomes more generic. All the cliches of cop thrillers are here (a dawn raid that involves a perp fleeing in their undies, a detective on the brink of retirement roped in for “one last case”) and courtroom proceedings grind the emotional narrative to a halt. The problem with having victims who are more interesting than their killer is that they must, necessarily, exit the scene. And then the drama is left with a vacuum.

Phelps, who has previously been responsible for turning Hercule Poirot into a gritty action hero, can create tense dialogue and emotional set pieces. “The craving,” Peter tells his vicar, in a heart-breaking scene of self-denial. “It’s a squalid witness for Christ.” But the more the drama proceeds into something run-of-the-mill (the third episode ends with Ben sinisterly peering out from behind prison bars) the grubbier these early passages feel. The Sixth Commandment may be dedicated to Peter and Ann, but their loneliness is merely a prop to gawp at this perverse manifestation of pure evil. Gripping, sure, but not sensitive.

And The Sixth Commandment aspires to sensitivity. Spall and Reid both give performances that beg the mercy of viewers, if not their captor. But Ben looms over events, immoral and unknowable. It has a gravitational effect, pulling the drama out of orbit. By the time the jury hears the evidence, the aspiration to justice for the victims has been replaced by the intoxicating propulsion of true crime. For a primetime show that started with so much promise, the dock is a rather tawdry place to end up.