Philip Glass’s 1983 Akhnaten is, I suppose, an opera. It has, after all, singers, an orchestra, chorus, staging, even arias. But Akhnaten diverges so far in so many different aspects from one’s standard expectations of opera that it’s hard to find a point of reference: nothing is what you expect. The results, however, make for utterly absorbing listening and – especially – viewing. Visually, Phelim McDermott’s new production for English National Opera never falls short of spectacular.

Glass’s music shares a quality with that of Bach: he is constantly setting up your ear to take it travelling in some direction where the endpoint is deeply satisfying. But the resemblance ends there: where Bach achieves his goals through the complexity of the counterpoint or of harmonic progression, Glass achieves them with repetition and simplicity. In Akhnaten, we never stray far from A minor and pretty much none of the music is without some repeating figure – most often in the foreground, sometimes in the background. But within those repeating figures lie ever-changing rhythms and combinations of instruments, and the relatively small number of harmonic shifts are compelling. Does repetitive have to mean boring? Absolutely not: after three hours, I was still entranced by the music.

Glass isn’t really interested in relating the details of the story of the Egyptian pharaoh who attempted to convert Egypt to a new monotheistic religion. “Akhnaten inherits the throne, he converts Egypt to monotheism, he retreats into self-imposed spiritual exile, the old order overthrow and kill him” just about sums up all the details that we find out. Glass is more interested in painting sonic pictures of the emotions surrounding the ideas and in the psychosexual aspects: Akhnaten is considered as the archetype of the Oedipus complex, and although he was married to Nefertiti, one of the great figures of female beauty in the ancient world, the depictions of him are notably gender-ambiguous, something of a point of focus in this staging. Glass brings out the emotions by varying the instrumentation and by blending voice types in a way that's quite unique. In this production, Anthony Roth Costanzo brings to the title role a voice that packs more raw power than any countertenor I can remember, while being weird, other-worldly and totally pure. He shows utter commitment to the role, and is ably supported by Emma Carrington’s Nefertiti and Rebecca Bottone’s Queen Tye: their duets and trios enfold us in warmth.

The ENO’s chorus are on strong form. Large tracts of the singing happens in ancient, forgotten languages, there are no surtitles, and anyway, I couldn’t understand much of the singing when it was in English. Inexplicably, this didn’t seem to matter: conductor Karen Kamensek weaves everything into a wonderful, ever shifting tapestry of sound. Such narratives as there is gets carried forward by spoken word, most notably by the imposing Zachary James.

Most of us have strong mental images of ancient Egypt. Tom Pye’s sets and Kevin Pollard’s costumes continually refer us to those images – the silhouetted views in profile, the heads of animal gods, the regal headdresses – without ever attempting to be “period costumes”. With the help of Bruno Poet’s lighting, they keep springing surprises on us: the scene in which Akhnaten is dressed for his coronation, from stark full frontal nakedness to orientalist baroque regalia of extreme opulence, surrounded by light rays to pick out the famous Egyptian sun god iconography, is just one of many visual delights; the great orb which shifts colours as Act II progresses is another. This is a production that is constantly watchable in the same way that Glass’s music is constantly fascinating to the ear.

Oh, and there’s juggling. A team of ten jugglers, to be precise, Gandini Juggling, who display great collective virtuosity with juggling clubs and balls varying in size from standard to gigantic. Opinions will vary as to “stroke of genius” or “overdone gimmick”: it worked for me. The juggling forms part of stage movement which is choreographed with the precision of a Swiss watch, and keeps holding our attention.

Before coming to Akhnaten, I could not have conceived of how an opera could have a single pace for the whole narrative – glacially slow – and none the less be utterly enthralling from beginning to end. And while I’m not at all convinced that I’ve taken in any of the underlying messages, this production has genuinely expanded my operatic horizons. A winner.