Miraculous as Jonathan Harvey’s music is, it seems reductive to call Wagner Dream an opera. Half the cast, after all, don’t even sing, but act: it’s a sincere meeting of music and theatre in a way that opera usually isn’t. Add to this director Pierre Audi’s sensitive but conceptually bold production, and the essential impression is of a dramatic multimedia artwork, which happens to crucially involve music – something of a Gesamtkunstwerk, perhaps.

It’s the last day of Wagner’s life. He rows with Cosima, suffers a heart attack, and hallucinates about an opera he never completed on a Buddhist theme. He dies. But with him, we hear the Buddhist opera – sung, unlike Wagner’s own domestic story. The opera Wagner dreams tells of the forbidden love of Pakati, a girl of low caste, and Ananda, a young monk. When Pakati pleads to the Buddha to have mercy on them, he eventually relents, by overturning the existing way of things and inviting Pakati to join the order, despite her gender and social status. Pakati and Ananda’s love must be chaste, but they will be companions in their faith.

It seems that part of both Wagner and Harvey’s fascination with this Buddhist tale stems from the very un-Wagnerian fate of its heroine, who finds redemption in life, unlike Isolde, Brünnhilde and so forth. The very concept of such a peaceable, generous opera by Wagner is not easy to digest, despite his great interest in Buddhism – as Harvey put it in a programme note, with reference to Parsifal: “Wagner’s racialism, nihilism and a hatred of the world distort Buddhist philosophy”. Hence, perhaps, Wagner’s dismissal of the work he dreams here: in a highpoint of Jean-Claude Carrière’s text, he reacts angrily to the way his vision ends, apparently seeking to distance himself from his own addled mind’s creation.

Not that the Buddhist opera sounds anything like Wagner – it’s an irony of Harvey’s music, presumably very deliberate, that the scenes in the Wagner household have a more Wagnerian harmonic soundscape than the opera he is meant to imagine. The Buddhist scenes are a vision of transcendence, something maybe just beyond Wagner’s reach.

But Harvey’s score is an astonishing, transcendent thing. The electronics meld assuredly with the live orchestra and are often made to function as evocations of the beyond; the sounds produced live, though, are no less compelling, especially in this strong rendition from the Welsh National Opera Orchestra and conductor Nicholas Collon. The scoring during the spoken scenes is a marvel of subtlety – too much so at the opening, in fact, where the actors’ booming tones overpower the soft musical backing – and the Buddhist music is beautiful, with one foot in pentatonicism and the other in brilliant, Stockhausen-esque mysticism. It’s all a study in consistency and effectiveness from Harvey, as opposed to a Wagnerian showpiece, but moments stand out especially: Ananda’s scintillating vision of the goddess; the well-integrated snatches of a Schubert piano piece we hear Cosima playing after a slight from Wagner.

Pierre Audi’s two-level staging, with the Wagners pacing around in grey below and the Buddhist story unfolding above them in bright colours, only ever adds to the experience, and the same is true for the stage and lighting design, both by Jean Kalman. Having the orchestra in plain view centre-stage – subverting Wagnerian practice – adds an enormous amount. So does the use of the languages German and Pali, which is completely new for WNO’s production (all previous performances have been in the original English throughout): this extra layer of remove from the action adds to the dreamlike ethos, and the switching between German and Pali highlights the remoteness of the two cultures.

The cast – both casts – excel. Actors Gerhard Brössner and Karin Giegerich are an appropriately unlovable Richard and Cosima, Richard unreasonable, capricious and pained; Cosima severe and hurt, but dutiful. Their very different counterparts Ananda (Robin Tritschler) and Pakati (Claire Booth) convince, with Booth relishing the opera’s most attractive, colourful vocal writing. Richard Wiegold brings grace and calmness to the role of Variochana, Wagner’s personal Buddha who guides him through his final moments; the Buddha himself, David Stout, radiates strength and warmth, both vocal and spiritual. Richard Angas as the old Brahmin sings well, but the character is weak: he is a conservative figure who vocally opposes change to the established Buddhist order, threatening to drag this otherworldly artwork into dialogue with mundane real-life issues regarding progressiveness in the church.

Overall, it’s a triumph, and WNO deserves the highest credit for being the first in the UK to stage this important piece – so clearly on a par with those recent triumphs of contemporary opera The Tempest, The Minotaur and Written on Skin. What a fine tribute as well to Jonathan Harvey, in the year following his death. With its deep-set spirituality and lively, fascinatingly angled dialogue with music’s past – not to mention its adept use of electronics – Wagner Dream encapsulates a lot of what is so special about this composer. Its spiritual message is, naturally, highly personal, and it’s difficult to appreciate the lesson it teaches: like a Zen koan, it doesn’t present answers so much as different, better questions. But can art ever offer us more than a glimpse of the beyond? It’s as far as Wagner ever got, for sure.