Gwyneth And The Green Knight, Theatr Brycheiniog, Brecon

Tender is the knight

Nestling amid Welsh lambs and rolling Beacons, Brecon has a new theatre. Theatr Brycheiniog, styled like an old malthouse and sited by its own canal basin, is as bewitching a venue as any in the principality.

The new auditorium with bright acoustics made a swell backdrop for a perfect new opera. Music Theatre Wales is invariably excellent; this show excels. Gwyneth and the Green Knight, newly commissioned from Lynne Plowman, is one of the nattiest new British operas in years. Plowman belongs with Judith Weir, Sally Beamish and the brilliant women composers' band whose achievements makes patronising redundant.

Gwyneth who? Like the footballing Asian lass with a yen to play for England in Gurinder Chadha's Bend it Like Beckham, Gwyneth is a societally incorrect, besmocked peasant wench who yearns to join King Arthur's Round Table. Disguised as a feller, she gets hired as Sir Gawain's squire and proves a far better bet than him. Enlisting the aid of the intermittently headless Green Knight and the ghastly boy Mordred, she routs Morgan le Fay and wins her fiefdom from the grimacing, knock-kneed Arthur (Richard Suart).

Is this adult opera, children's adventure or pantomime? The Greeks had a term for it. Once a trilogy's worth of Oresteian, Oedipean or Trojan blood had flowed, they tacked on a fourth "Satyr" play, a spoof sending up the whole grisly tale. Slot Birtwistle's Gawain – Plowman's jingly score actually has quite a bit in common with his works – into John McCabe's ballet Arthur, append Gwyneth, and you have a classic Attic tetralogy. This was opera without concessions, for everyone. A six-year-old adored it; so did a sprightly septuagenarian.

Why so good? Well, Plowman's brilliantly illustrative music – pointillist brass, bristling strings, quizzical percussion, mocking pizzicati – never falters. Add Martin Riley's pithily funny rhymed libretto and you have another Knussen's Where the Wild Things Are.

The cast is one to kill for. Suart has a classic huff-and-puff role as a useless "not me, luv'" Arthur; Charles Johnston launches the show ringingly as the bluff father who wants his Malory-fed lass to marry the local cowhand. The tenor Ashley Catlin is not so much Gawain as Paul Merton at the Round Table. Sinead Pratschke is made for Gwyneth, doubly disarming when she reveals her Viola secret to save her master. Richard Wiegold's ho-ho'ing Green Knight is John Tomlinson meets Graham Chapman. Kathryn Turpin and James Taylor as Morgan and a vicious chorister Mordred are deliciously appalling.

Simon Banham's designs plus props-maker Laura Martin's paraphernalia seemed perfect; Ace McCarron is a genius with the lighting. Gwyneth is a smash-hit for MTW's gifted two Michaels, director McCarthy and conductor Rafferty. Shoo it across the border soon, please.

Cardiff (ends tomorrow), Mold (23, 24 April) and Swansea (25, 26 April)