Der Ring des Nibelungen, Royal Opera House, London

Here, at last, is the Covent Garden 'Ring' in full. Keith Warner's 16-hour marathon haunts the listener long after the final curtain
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From the first dark note of Das Rheingold to the final shimmer of Götterdämmerung, time slips and slides narcotically in Der Ring des Nibelungen. What is one to make of a 16-hour-long work that features dwarves, dragons, giants and talking birds yet critiques theism, technology, family and capitalism? Heard separately, the four operas of the Ring can make you feel like Dorothy in The Wizard of Oz: menaced and excited by a world of monsters, half-truths and miraculous transformations. Heard in one week, as Wagner intended, they instead make you feel like poor Judy Garland must have felt during the filming of The Wizard of Oz: alternately stimulated and sedated, stupefied yet unable to sleep, a walking chemical ex-periment, half-adult, half-child.

Having seen each part of the Scottish Opera Ring, watched English National Opera's semi-staged performances and subsequent stagings, and followed all four of Keith Warner's Royal Opera House productions, I felt reasonably well-prepared for Covent Garden's complete cycle. Like Siegfried, I was over-confident.

On an average night at the opera, you might leave with one or two ear-worms. During the Ring, you go home with an infestation. Bullied by Wagner's undisciplined genius, jet-lagged from the distortion of clock-time, you begin to hear voices. Brush your teeth and Fasolt and Fafner stomp into the bathroom. Hang up the laundry and the Woodbird is perched on your shoulder. Do the school run and Alberich is issuing the Liebesfluch from the car behind you. Go to the supermarket and there's Brünnhilde in the queue: "Ewig war ich, Ewig bin ich". These symptoms could be those of an infatuation. But experiencing the Ring in one week also alerts you to its faults.

That Wagner's tetralogy was composed over more than two decades shows. There are times, most particularly in Siegfried, when the leitmotifs are less a manifestation of an overarching vision than a rudimentary filing system, thematic Post-It notes in a disorganised narrative. The sole musical joke – Siegfried's attempt to mimic the Woodbird – would have been cut had it been written by any other composer. Only Act III of Die Walküre approaches Tristan und Isolde in its measured intensity, its perfect balance of spectacle and secrecy, and for every transition in which the scoring is cruelly intoxicating – those pinched, dazzled woodwind chords – there are as many clunking inconsistencies. No wonder Wagner's most fervent acolytes prefer to hear the Ring with its lumps and bumps smoothed out, as, it seems, does Antonio Pappano.

Of the many shifts of emphasis since Das Rheingold opened in 2004, the most profound is in Pappano's conducting. Once episodically brilliant, vigorously contrasted, almost improvisatory, his Ring has become a grand, glamorous curve. Something of the old immediacy remains in Siegfried and Götterdämmerung, most especially in Siegfried's Rhine Journey and the Funeral March, but in Das Rheingold and Die Walküre local colour is secondary to narrative drive. Though sensitive to the delicately phrased trios of the Rhinemaidens (Sarah Fox, Heather Shipp, Sarah Castle) and Norns (Catherine Wyn-Rogers, Yvonne Howard, Marina Poplavskaya), there is a loss of intimacy too – perhaps because the only cast-member who could sing Wagner as lieder has left the production.

Bryn Terfel's replacement as Wotan, John Tomlinson, may not have the younger singer's effortless beauty of sound, but he has enabled Pappano and Warner to present a more unified Ring, turning a star-vehicle into an ensemble piece. Previously spiked with sexual boredom, the relationship between Wotan and Fricka (Rosalind Plowright) is initially cosy, all chess and knitting. Which makes their next encounter, bitterly squabbling over the fate of Siegmund (Simon O'Neill) and Sieglinde (Eva-Maria Westbroek), far sharper. If Tomlinson inclines to bluster, he is both a persuasive tragedian and a confident, playful comedian: a team-player whose exchanges with Alberich (Peter Sidhom), Brünnhilde (Lisa Gasteen), and Siegfried (John Treleaven) have a poignancy and pathos that comes with real, rather than faked, age. For a god, he is very human.

As before, Warner's personenregie is one of the most satisfying aspects of this cycle. If Kurt Rydl's Hagen is too enigmatic to arouse more than antipathy, Sidhom's Alberich is a harrowing portrait of desperation, ambition and rage. Though Gasteen and Treleaven squall at the top, and neither cuts a heroic figure when moving at speed, they are more than battleaxe and buffoon. For vocal radiance, however, they are overshadowed by O'Neill and Westbroek, who are perhaps a decade away from being a sensational Siegfried and Brünnhilde. Impressive too are the gore-smeared Valkyries, Will Hartmann, Peter Coleman-Wright and Emily Magee's drowsy, decadent gods, Franz-Josef Selig's sympathetic Fasolt, Gerhard Siegel's energetic Mime, Philip Langridge's twitchy Loge. Indeed, this Ring is rich in comedy: satirical, broad and tender.

Still, the simplest and most serious images in Warner's production remain the strongest, and the moments where these coincide with echoes of Pappano's original, highly textured approach are, for me, the best. Though I've spent an absurd amount of time wondering whether Alberich's aeroplane is a reference to Charles Lindbergh's experiments in "the science of immortality", what has stayed with me is the designer Stefanos Lazaridis's white wall. Pushed against the imprecations of the Valkyries by Wotan, spun around as the Wanderer rages against his impotence, flooded with light at Brünnhilde's awakening, tossed on the waters of the Rhine, dragged from the back of the stage as a wedding bier for the betrayed Brünnhilde, this hydraulic abstraction has proved more eloquent than any number of prosthetic limbs and lobotomised slaves. For hell, in the end, isn't Nibelheim. It's powerlessness, regret, and guilt.

Royal Opera House (020 7304 4000), to 2 Nov

Need to know

'Der ring des Nibelungen' was first performed in 1876 but was begun in 1848 in response to civil unrest in Europe. Based on Nordic myth and informed by the philosophical writings of Ludwig Feuerbach, it is the longest work in opera and Richard Wagner, above, considered it his greatest achievement. It's also the costliest project an opera company can undertake. Keith Warner's production is the seventh staging by the Royal Opera House.

Further reading Jean-Jacques Nattiez's psychoanalytical study 'Wagner Androgyne' (Princeton) price £24.02