I Am Always With You:The Sixth Commandment.

The Sixth Commandment (BBC1/iPlayer, written and created by Sarah Phelps and directed by Saul Dibb) is true crime for grown ups. It's incredibly sensitively handled, there's no gore and there's no violence, but yet it's utterly compelling from the first to last minute. You'll struggle not to binge watch a programme that is, essentially, a masterclass in how to tackle this kind of subject matter.

Peter Farquhar (Timothy Spall) is an inspirational teacher at Stowe school in Buckinghamshire who lives in Maids Moreton, a quiet village nearby. A much loved man, he is reaching retirement age and is working on a book. He is also gay but he's so repressed, and so afraid the church he attends and serves as a lay preacher for will disown him, that he never acts on his impulses. He won't even look at pornography. He simply desires "to hold and to be held". He can cope with the celibacy but he is desperately lonely and years for intimacy.

Into his life comes charismatic, confident, and handsome student Ben Field (Eanna Hardwicke). Peter sees Ben as clever, enthusiastic, and delightful and he can barely stop talking about him. Ben starts to worm his way into Peter's life and Peter, happily, lets him do so. Ben's friend Martyn (Conor MacNeill) moves in to Peter's house as a lodger and Ben writes Peter a poem and begins to attend church with him.

Soon enough, Ben tells Peter he's fallen in love with him and Peter replies in kind. But the title of the programme should make it abundantly clear that all will not go smoothly. It doesn't. Peter's bliss, Peter's happiness, at finally finding love will not last forever. Or very long at all.

Ben brings Peter breakfast in bed while Peter changes his will to make Ben the main beneficiary. Then, as quickly as he fell in love, Peter falls ill and falls down the stairs. Peter does a lot of falling. But things soon get much much worse. Things get as bad as they possibly can.

Ann Moore-Martin (Anne Reid) is a near neighbour of Peter and Ben. A retired headteacher and, like Peter, a deeply religious person, she lives alone with her little dog Rosie. When Rosie dies, Ben sends Ann a card and soon he begins to inveigle his way into her life too. His creepy behaviour around older people put me in mind of Sting's Martin Taylor in Dennis Potter's Brimstone And Treacle.

Despite the huge age gap they end up in a relationship of sorts and it's not long before Ann, like Peter before her, starts having dizzy spells. Ann's niece, Ann-Marie Blake (Annabel Scholey), becomes suspicious of Ben's intentions and behaviour but Ann-Marie's husband, Simon (Ben Bailey Smith), dismisses her concerns. Ben's only being neighbourly, helping out around the house, giving Ann lifts to church. It's what any good neighbour would do.

But it's apparent to us, the viewers, from very early on that Ben's motivations are not benevolent. The drama comes in waiting for others to see that, wondering when scales will fall from eyes. Ben's malicious behaviour is made all the more disquieting by the fact all his interactions are carried out quietly, serenely, with all the grace of a deeply spiritual man. Or, indeed, a cult leader. When Peter's brother, Ian (Adrian Rawlins), and his wife, Sue (Amanda Root), are confronted with the terrible reality of the situation your heart sinks with theirs.

And what of Martyn? Is he Ben's accomplice in all this or is he simply another one of his victims? Has he been taken in by Ben? In many ways, Martyn is crucial to understanding what's going on but Martyn himself, a failed magician, doesn't give much away.

To get to the bottom of it all, we're taken on a tense, chilling, eerie, sometimes even frightening journey that takes in God delusions, communion wafers, creepy messages written on mirrors, smoothies, swingball, giddy spells, University Challenge, gaslighting, and, ultimately, Thames Valley Police in the form of DS Natalie Golding (Anna Crilly) and DCI Mark Glover (Jonathan Aris).

About halfway through the drama partly becomes a police procedural and, after that, we move to the court room. That's fine. I love police procedurals and court room dramas and here they are both done exceptionally well. That's testament to an absolutely brilliant cast (honourable mention to Sheila Hancock who plays another of Peter's neighbours, Liz) and outstanding central performances from Spall, Hardwicke, and Reid. My friend Adam described Spall, not unreasonably, as "a national treasure" and it's true. If he was a building he'd have been listed by now.

The Sixth Commandment is beautifully shot, there is space for the story to breathe and for the characters to develop but it's never too slow. In fact it's just the right pace. Almost every scene is permeated with a foreboding atmosphere that seems very much in keeping with its dark subject matter.

Ultimately, The Sixth Commandment deals with a man with a messiah complex so deadly it has turned him into a monster. A man who prayed with, and then preyed on, some of the kindest, loneliest, and most vulnerable people he could find. By the end you'll be desperate for justice to be served. But will it be?