Ten half-dead bodies spew their guts all over the stage. The killer, a grotesque man-bull, staggers around in bloodlust. A gloating chorus bays from the tier of a gore-smeared bullring.

The orchestra lurches into a macabre dance of death, propelled by jagged percussion blitzes and searing woodwind discords. Screeching harpies swoop on the bodies and rip out their hearts.

And that’s just the Act I finale. The Sound of Music it ain’t. In fact it could probably only have come from one composer. Harrison Birtwistle has been creating labyrinths of sound all his life. So it was probably only a matter of time before he turned the Greek myth of the Minotaur trapped in a labyrinth into an opera, which received its world premiere at the Royal Opera last night.

The story tells of the half-man, half-beast Minotaur, imprisoned by his stepfather, the Cretan King Minos. He slaughters Athenian virgins for breakfast, until he is in turn slain by the Athenian hero Theseus, with a little practical help (a ball of twine, for navigating the labyrinth) from the Minotaur’s own half-sister, Ariadne. She’s eager to escape her own labyrinth of sexual frustration by eloping with Theseus.

But no Birtwistle opera is straightforward story-telling. What he and his librettist, David Harsent, seem intent on exploring here are the ambiguities inherent in our too-neat separation of behaviour into “bestial” and “human”. As the Minotaur’s monologue reminds us, it’s humans who rage, lust and inflict cruelty. The Minotaur, Theseus and Ariadne are all equally trapped by the actions of their parents and the callous gods. And at the end it’s the man-bull who turns out to the most vulnerable — the most human.

All that is interesting. And the opera couldn’t have been more strongly cast. As usual, you can’t take your eyes off John Tomlinson in the title role, even when he’s encased in a huge bull’s head. He captures exactly the sense of a bewildered, caged animal, and his singing is terrifically baleful. But Christine Rice is just as characterful as Ariadne and Johan Reuter conveys effectively the feeling that Theseus will dump this whining woman as soon as he reaches the next island.

Yet The Minotaur rarely gripped me as some of Birtwistle’s operas have done. There are amazing orchestral effects and the voice-writing is more expressive than in Birtwistle’s earlier scream-fests.

But there’s little evidence here of him breaking genuine new ground. And despite Stephen Langridge’s efficient staging, Alison Chitty’s elegantly sparse designs and Antonio Pappano’s well-paced conducting, many scenes feel very ponderous. After three hours, like Theseus, you may be reaching for that twine to lead you out of the labyrinth.

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