When you discover your bride-to-be in another man's bed on the eve of your wedding day, the excuse "Darling, I must have been sleepwalking" rates somewhere near the same level of credibility as "Sir, the dog ate my homework." But hey, this is opera, and bel canto opera at that. You didn't come to Bellini's La Sonnambula for the story and the drama: you came here for the singing.

Most particularly, you came here for the prima donna, and Eglise Gutiérrez delivered a performance that was worth the visit. The role of Amina was originally written for Bellini's favourite diva Giuditta Pasta, whose voice is now categorised as a "soprano sfogato", basically a mezzo-soprano whose voice could stretch up to a soprano's high notes. Gutiérrez sounded wonderfully warm and rich in the low parts of her register, sliding effortlessly up into high notes in intricate coloratura, totally confident at all points in between. She produced plenty of trills and flourishes when required while keeping to the purity of Bellini's vocal line at all times: it was a superb vocal performance.

The supporting female roles were well acted and sung, Elena Xanthoudakis resplendent in fuchsia as the bitchy Lisa, and Elisabeth Sikora the archetype of the heroic Italian mother. Sadly, the two leading men didn't reach the same level. Michele Pertusi was genial as the Count, but didn't command the house. As Elvino, Celso Albelo was clear, in tune and followed all the lines perfectly. But it felt restrained and passionless: when Elvino yelled "Va, traditrice" at Amina, all I heard was a nice melodic line, not the fury of a man spurning his treacherous lover; when he sang to Amina that "non sei cancellata del mio cor" (you still have a place in my heart), it was an elegant phrase rather than the agonies of a man torn between love and jealousy.

It all wasn't helped by the orchestra, under the baton of Daniel Oren, who turned in a performance that felt rather leaden. In Bellini's tragic operas, the music is often too bouncy for its own good, but La Sonnambula is a romantic comedy, and it can do with all the bounce that a conductor can inject: whether from slowish tempi or lack of accenting, Oren failed to provide the required energy. Even the overture failed to come off: it has the fun and unusual feature of being a conversation between the main orchestra and a "sul palco" band on-stage, but the latter was so quiet and muffled by the Royal Opera House's voluminous velvet curtains that they were barely audible.

Marco Arturo Marelli's 2002 set was pure eye candy. We are in the Swiss Alps as per the original, but rather than in a village by Teresa's windmill, we are at a wedding banquet in the atrium/ballroom of a very smart hotel, with the snowy mountain visible through the windows, gorgeous art deco ironwork and elegant evening dress all round. It's all lovely to gaze upon, although achieved at the expense of some hideous continuity errors: after all, the whole point of the first act is that it's a betrothal ceremony and we're not even sure if the wedding will take place at all. Worse still, towards the end of the act, the impact of the story is rather lost when Amina is peacefully asleep in front of a banqueting table rather than on a bed in Rodolfo's room.

La Sonnambula is labelled as an "opera semiseria", an opera with elements of both comedy and pathos. Marelli sets may have supported the frothy romantic comedy aspect (he also directed the original 2002 production), but Oren and revival director Andreas Leisner seem to have taken the opera rather seriously, which, to my mind, is a mistake: the story isn't strong enough to support it, nor does Bellini's music, which is basically cheerful and bubbly. La Sonnambula works best when it fizzes and sparkles, and last night's production didn't - with the exception of the female singers and particularly of Eglise Gutiérrez, who gave us an Amina to remember.