Stuart MacRae's new opera, The Assassin Tree, is this year's headline new music event at the international festival, an ambitious collaboration from MacRae, poet Simon Armitage, and directors Emio Greco and Pieter C Scholten. On paper, The Assassin Tree is full of impressive ideas but, on stage, the effect is somehow less than the sum of its parts.

Individually, each element of the production is coherent and sometimes striking: Greco's stylised sets create the setting of the goddess Diana's sacred grove, where Gillian Keith's Diana is guarded by a priest, sung by Paul Whelan. The Priest is both privileged and cursed, since he has to protect the goddess's golden bough from thieving slaves who can win their freedom by stealing a leaf from the tree, and anyone who kills him automatically takes his place as Diana's guardian. The tree is symbolised by a stack of lights and scaffolding; a strange, moon-like disc at the back of the stage becomes the emotional barometer of the piece, its colours changing with the mood of the characters.

Gillian Keith is a vocally and physically powerful Diana, and she and Whelan are masters of the mysterious choreography that Greco creates for them, a series of semaphore-like gestures that dramatise their arcane characters, making them part myth and part human. MacRae's music attempts the same trick, dramatising Diana on one hand as a cold-hearted goddess and on the other as the embodiment of sensuality, especially when Peter van Hulle's Slave attempts to steal one of the precious leaves.

Scored for 15-piece ensemble - the Britten Sinfonia, conducted by Garry Walker - the music has all the intensity and focus of MacRae's recent work, with its ritualistic power and elemental energy, especially in the dissonant fanfare of the opening. The problem is that, in creating the mythical realm of Diana's grove, the human side of the story is lost. MacRae's uncompromising vocal lines are expertly negotiated by the singers, but Armitage's poetic, allusive text is scarcely audible, and the delicate symbolism of the libretto is undermined by its musical treatment. The Assassin Tree is a bold piece of music theatre, but it fails to alchemise the individual elements of its text, music, and staging into a convincing dramatic experience.

· At the Royal Opera House, London WC2, from September 6 to 8. Box office: 020-7304 4000