The Sixth Commandment review – as immaculate a piece of TV as you will ever see

You may think you are beyond being harrowed at this point; that age, experience and daily headlines testifying to the relentlessly awful nature of the world have created a callus on your soul that protects you from such profound disturbance. Especially via such a quotidian, unassuming – essentially friendly – form as television.

Then along comes something like The Sixth Commandment that – over the course of its four immaculate episodes – shaves that callus off scene by scene and reminds you of what television, people and your heart can still do.

Written by Sarah Phelps and directed by Saul Dibb, it tells the true story of retired Stowe schoolmaster Peter Farquhar (Timothy Spall) and his neighbour Ann Moore-Martin (Anne Reid) who, one after the other, were befriended by a young churchwarden called Ben Field (Éanna Hardwicke), who made himself indispensable to their lives and who, after they changed their wills to make him the main beneficiary, murdered Peter and attempted to murder Ann. Before that he subjected them both – especially Peter – to repeated poisonings and various forms of mental torture for no other reason than for sport.

Phelps is best known for A Very British Scandal and her masterly adaptations of various Agatha Christie novels, but here she applies all her considerable craft to making something much more subtle and even more savage, in its own quiet, terrible way. The layers of horror in Peter’s story, which dominates the first hour, pile on inexorably until watching becomes almost unbearable. It is a study in the power of faith, loneliness and the endless vulnerability of humanity – especially that of the good, innocent and unworldly to anyone free of moral shackles.

Peter is a homosexual man who has, because of his faith and his temperament, barely ever acted on his impulses. “Even my deviance is pathetic,” he tells his priest and counsellor. We watch helplessly as Ben, with the effortless grace of a born predator, coolly takes the measure of this good and suffering man, his yearning for any sort of companionship (he will settle for so little) and his self-loathing, and makes his move. Gentle affection, some poetry, a professed love of Christ, and Peter is suffused with happiness at having found his soulmate. The poison in their relationship is at first metaphorical – as Ben makes himself central to Peter’s world and isolates him from the rest of it while sowing seeds about Peter’s (nonexistent) drink problem before casting himself as the young saviour – and then literal.

Spall is as good as – or perhaps even better than – you have ever seen him, which is not something to be said lightly. Blessed by some beautiful writing and direction, he gives us a wonderful, compelling portrait of what can easily be the most boring of subjects: a good man. Peter – a beloved teacher, still guest-lecturing at a university after retiring (which is how he meets Ben) – is clever, funny, kind but tormented by what he sees as his terrible weakness. His faith is his bedrock but church teachings mean he can never be reconciled to his homosexuality. It is another one of The Sixth Commandment’s many strengths that it never sneers at faith. In the coming episodes, with Reid as Ann in particular, it honours its importance to people and the comfort it can bring, even as Ben perverts all that it should be.

Balancing the performance and narrative scales is Hardwicke as Ben. Evil characters can be just as one-note and boring as the good, but Hardwicke manages to infuse Ben with such a disconcerting blend of calculation, charm and quiet, almost hidden glee in the harm he inflicts that it is impossible to look away.

You will want to. It is harrowing. The appearance of the snake in the garden of these good and godly people is terrible. Ben murders so much before he kills anyone. And beyond him and his victims are all those who love them. Everyone in the supporting cast is as brilliant as the main protagonists, including Annabel Scholey as Ann’s devoted niece who is almost broken by the unrepentant malevolence of the man who infiltrated her aunt’s life; Conor MacNeill as Ben’s guilt-ridden accomplice and victim Martyn; and Anna Crilly, Jonathan Aris and James Harkness as the police investigators who eventually trap Ben. Altogether it is as fine a piece of television as you will ever see. Clearly intensely researched and forged with love and respect, it also stands – and I hope their friends and families agree – as an equally fine memorial to Peter Farquhar and Ann Moore-Martin, in all their unsullied goodness.

• The Sixth Commandment aired on BBC One and is now on iPlayer