The Assassin Tree, Royal Lyceum Theatre, Edinburgh <!-- none onestar twostar threestar fivestar -->

At last, a thoroughly successful new Scottish opera. Not that Stuart MacRae's The Assassin Tree is an opera in the normal sense: it's more a piece of music theatre, recalling Britten's church parables and some of the works of Maxwell Davies. Based on the story of the king of the wood in Frazer's The Golden Bough, it epitomises the chemistry of sexual relations in portraying the goddess Diana as a femme fatale, leading men to their deaths as each king-priest is murdered by his successor.

A major part of the success is due to the author of the text, Simon Armitage. The art of the opera librettist is not much in demand nowadays, and as a result many operas are spoilt by clumsy texts, the work of novelists and playwrights. Armitage constructs a neat pattern of action, simple and ingenious, with only four characters (the whole work lasts just over an hour). Relinquishing his usual streetwise style, he elevates the language sufficiently to suggest mythic nobility, without losing familiarity and fluency. There is hardly any true dialogue; the characters sing mainly to themselves.

MacRae sets this many-layered, psychological drama in a continuous recitative or parlando, the instruments (the glittering Britten Sinfonia under Garry Walker) providing a caressing or hectic commentary. The style is doggedly atonal but authentic and lucid, coming from a vivid aural imagination. It seems that the composer's early, rather gentle manner has been superseded by a high seriousness.

The impression of formal ritual is stressed by a hieratic production style, centring on dance and mime. Emio Greco and Pieter C Scholten choreograph even the smallest movement; often the people shadow each other's gestures exactly, as though in a mirror. The work almost becomes a ballet. They have also designed the set, with Diana's tree a tall lighting gantry, the trees of the wood narrow silver flats, all atmospherically lit by Henk Danner. Above the stage, a great moon (Diana's planet) is progressively bloodied.

As well as his fine librettist, MacRae has been lucky to find a superb cast. Above all, Gillian Keith makes a girlish Diana, her ruthlessness ("All my husbands are murdered; all my lovers have killed") tempered by a creamy lyric voice, her movement partly Isadora Duncan, partly torch-song. Her three menfolk are sharply distinguished: the baritone Paul Whelan is a weary king-priest, a husband whose love has turned to routine, with a capacious voice and a figure of tall dignity. Her first suitor, a slave (Peter Van Hulle, a bright-sounding tenor), is foolhardy and priapic and easily overcome. His successor - another tenor, Colin Ainsworth, firm and commanding - kills the king by a ruse, until the final terrible denouement, which I will follow the programme-note in concealing.

The opera was a co-production of the Festival, the Royal Opera House and Scottish Opera, and was composed while MacRae was the Festival Creative Fellow in Edinburgh University. It will be seen in London next month, and afterwards in the repertoire of Scottish Opera. As with some of its predecessors in the music-theatre canon, its layers of meaning will provoke much discussion. Armitage himself sees it as an allegory of nature herself. But it is a simpler piece than some of the works of Britten. Diana is a temptress, a destroyer of men, like Carmen or Turandot; the suitors are lured by her proud beauty into self-immolation. It's a familiar operatic theme, especially in operas by male composers.

The Assassin Tree is at the Linbury Studio Theatre, Royal Opera House, London, 6 to 8 September (020-7340 4000)