Christoph Marthaler is a hugely significant figure on the European theatre scene, and his UK directoral debut at the Royal Opera House, with King Size at the Linbury, has been widely anticipated. There is no doubt that Marthaler's directoral approach is interesting: a hotchpotch of intimate naturalism (characters getting dressed and undressed on stage), formal theatricality and deliberate absurdism. It's not cataclysmically avant garde, but it is original. The overall effect is bewildering, but entirely benign: at one point, a character approaches a plant with a watering-can, stands and considers for a while, then retreats. The joke seems to be, quite simply, that he doesn't water the plant.

At least, I think that was the joke. King Size relies on this variety of very gentle - dare I say, rather Swiss - humour, which is charming in its own way, but doesn't come across naturally to a London audience, and we may need another chance to fall in love with Marthaler properly: while we giggled obediently there, and even hysterically in other places, I can't be sure that we were laughing altogether where Marthaler wanted us to all the time. The production, originally from Theater Basel, is intriguing, but feels oddly rootless, moving from one understated, vaguely humourous situation to another without plot, logic or even explanation of character. As a result, the audience is rather left to flounder, or simply enjoy what's happening. It was all a bit like going on a first date with someone you find hugely attractive, having some fun, yet never quite finding the subject of conversation which brings you closer.

Duri Bischoff's design of a classic hotel room references Richard Jones in its heavily printed wallpaper, carpet, strong sense of colour and immaculate layout (not to mention uniforms for some characters – not quite the whole bingo card, but getting there). More surreal elements, like the mini-bar lodged high out of reach in a fitted wardrobe, also recall Jones. Singers hide in cupboards, pop out from under the bed, and visit the en suite bathroom (complete with fart noises and flushing), as the room becomes progressively inhabited by four characters – or, at least, four performers, who either play several characters, or play single characters enacting further characters in dreams; I couldn't quite untangle where the dreams began or ended, or what the dreams were, or who the characters were supposed to be. Luckily, they are named in the programme.  

Tora Augestad, a tall and magnificent soprano, performs a series of songs in a series of costume changes which do not help to elucidate who she is at all. However, she is credited as the King's Daughter, a reference to her final, lovely "Es waren zwei Königskinder", a German folk duet which she sings with childlike simplicity and delicacy. For me, Augestad is the star of the evening: her delivery of Dowland's "Come, heavy sleep" from underneath the bed is an absolute showstopper, lyrical and plangent; her blowsy, jazzy approach to the more modern numbers is delightful. Augestad also has innate comic talent: as the evening goes on, her sense of humour shows ever more strongly in ironic dance moves, witty gestures and some deliberately hyperbolic singing. When you feel lost in the piece, Augestad's calm conviction and clear enjoyment of her role reassures you that it's still ok to laugh.

Augestad's duet partner for much of the evening is the diminutive and distinctive Michael von der Heide, termed the King's Son for perhaps the same Königskinder duet. Von der Heide is a nifty dancer, again communicating an extremely playful relish in his performance to the confused, delighted audience; his tenor sounds more West End than Royal Opera most of the time, but he delivers pop songs with sheen and style. The climax of his performance was his superlative rendition of Boby Lapointe's "Méli mélodie", a fiendish musical tongue-twister which von der Heide delivered with soft-toned, bell-like, awesomely impressive clarity.

The Polish-born actress Nikola Weisse plays Alma Lieder-Abend, an elderly lady who walks through the hotel room periodically, usually in silence. Occasionally stopping to meditate verbally on the process of ageing, Weisse is tasked with the most absurd elements of the piece: Marthaler has her eat cold spaghetti from her handbag using the complimentary backscratcher, an oddly unsettling act to watch, and also fight patiently with an old-fashioned music stand (which drew gusts of sympathetic chortling). Weisse also gets the dubious pleasure of reading out the nonsensical lyrics to Lapointe's "Méli mélodie" just before von der Heide sings them: the point seems to be that it makes more sense as a song. It certainly does. Weisse's presence is atmospheric, and might perhaps have provided the dark backbone this piece really needs, but we are so thoroughly disorientated that we often laugh when, perhaps, we ought to be moved by her.

Bendix Dethleffsen provides much of the keyboard accompaniment (a piano and keyboard are used) as Detleff Cohn-Bendix, as well as some super-soft pianissimo singing, which was sometimes almost lost altogether in the dark hole of the Linbury. Dethleffsen appears to be in charge of proceedings, seeming to conduct his fellow singers from the piano at times, and playing along with various stage antics.

The musical amuse-bouches on offer are, like the rest of the piece, charming and yet not profound. Sometimes this is triumphant: taking the opening of the Vorspiel from Tristan und Isolde, then segueing suddenly into the Jackson 5's "I'll be there", is genuinely hilarious. At other times, it all just feels like an hallucination, moving from one odd, lovely song to another with connections so subtle or obscure that they were certainly lost on me. The singing itself sounds generally more relaxed than we usually hear at the Royal Opera, and quieter, which does add a nice sense that we are eavesdropping on these singers as they revel in their own talent in rather a private and special way. Pop songs, particularly, become naturally comic in effect, as they do just sound like a joke beside the more serious music on offer. In the end, I decided, it is rather hard to attach meaning to any part of King Size, and you can waste time and energy trying: or you can just aim to enjoy it, moment to moment, and hope that next time Marthaler comes back to the table bringing something with more compelling narrative force.