Punch And Judy, Royal Opera House: Linbury Studio, London

Still murderous after all these years. Harrison Birtwistle and Stephen Pruslin's Punch and Judy is 40 years old. Mr Punch and his unfortunate wife go back much further, of course, but this extraordinary work was perhaps the defining moment when the Theatre of Cruelty set its mark on opera. That moment occurred in Jubilee Hall, Aldeburgh, in June 1968, when even our most astute and receptive of music-theatre practitioners, Benjamin Britten, found it just too, too much. It is, of course – but then nightmares are, aren't they?

And as Music Theatre Wales scarily remind us in the first of two London stagings of this incorrigible masterpiece within weeks (the second is by ENO at the Young Vic in April), the nightmare makes children of us all. No day out at the seaside would ever have been complete without the ubiquitous Punch and Judy show. So here we are back on the esplanade. There are deck chairs for the adults – the director Michael McCarthy symbolically lays on a couple of those – but, like children, do we question what drives Punch to take a blow-torch to his baby in the first five minutes of the show? There is no rhyme nor reason for the act – so we laugh. Isn't that what the young thugs on YouTube do? Isn't that what happy-slapping videos are all about? Unsettling, isn't it?

At least Pruslin and Birtwistle present the possibility of redemption for Punch's actions. Birtwistle's love of myth and ritual casts his tragicomedy as a kind of Passion, where his Puppet Master or Evangelist, Choregos (the wiry, Dali-like Jeremy Huw Williams) consecrates "the sacrament of murder and sadism" on "the altar of pain".

Four nooses ominously descend, one for each of Punch's victims, but his odyssey is the search for love, and each time he woos and is rejected by Pretty Polly (Allison Bell, whose squeals of delight and irritation are like Mozart's Queen of the Night on helium), Birtwistle and Pruslin offer a chorale: "Weep out your unfathomable and inexpressible sorrow."

These words are set to music (a quartet) of ineffable beauty – though, of course, it is not the beauty that stays with you on your journey home. No, you'll be hearing Gwion Thomas's orgasmic whooping for joy as he, Punch, slashes and burns and beats his way into immortality. That and the compendium of shrieks and guffaws and cartoonish exclamations from a brilliantly accomplished 15-strong instrumental ensemble under Michael Rafferty. They are as much characters in this "quest" opera as those with names.

Punch does get away with murder, and he does win Pretty Polly, but even as the company are celebrating with a dance around the maypole, we are reminding ourselves of what happened to Judy, and fully expecting that it will happen again. Mr Punch is who he is – and he was well named.