Fidelio, Beethoven's only opera, is on the most universal of themes: the oppression of the innocent by the powerful and the virtue of the struggle for a loved one. It could be set at any time in any place, from Ancient Rome or China to our modern world. Jürgen Flimm's 2007 production, revived for Covent Garden, is timeless and placeless: only the weapons and costumes mark it in modern times, with a hint of "fall of Saddam Hussein" when workmen dismantle Don Pizarro's equestrian statue at the end, but we could be in any military prison anywhere in the world.

Fidelio is a strange hybrid. Partly, it's a serious angst-ridden grand opera, with our heroine Leonore saving her husband Florestan against all the odds, disguising herself as a man and getting a job in the gaol of the evil governor Don Pizarro. But this high-minded tale of heroism in marital love is woven into a fairly standard comic opera theme in which the gaoler's daughter Marzelline falls in love with our cross-dressed heroine, and is most discomfited when she eventually discovers her mistake.

Musically, Beethoven makes the hybrid work, and Mark Elder did a fine job of bringing out the drama and beauty in the music. The ROH orchestra got off to a slightly rocky start in the overture, in which a pacy tempo and good dynamics were spoilt by several fluffs and late entries, but they hit their stride shortly afterwards. By the middle of the first act, the blend of voices and instruments was quite entrancing, especially some of the woodwind lines. The music for Fidelio demands a degree of precision from the orchestra that's rare in opera, and Elder and the ROH orchestra delivered it. The opera is also notable for its ensemble singing, and here again, the production shone. The prisoners chorus in Act I, when Leonore unlocks their cells and they see the sunshine, was glorious, and nearly matched by the choral celebrations of marital virtue at the end of Act II. But the highlight of the opera, perhaps surprisingly, is a semi-comic number: the Act I quartet in which Leonore, Marzelline, her father Rocco and her spurned boyfriend Jaquino sing at cross purposes about their circumstances, is a stunning piece of vocal writing.

While the ensemble singing was immaculate throughout, solo performances were more mixed. The pick of the singers, I thought, were the father-daughter pairing of Rocco and Marzelline. Kurt Rydl sang Rocco with authority and a gorgeously rich bass timbre, and made us believe in the character: Rocco is a basically good man who is doing a dreadful job and lacks the moral courage to extract himself from it. Elizabeth Watts, who made her Covent Garden debut only last year, was a delight to listen to and showed the best of her lieder-singing talents to give us a Marzelline who is pretty, charming and heartfelt. Watts's voice contrasted with Nina Stemme's fuller, more heroic voice in a way that was well matched to the roles, although in her solo arias, Stemme didn't quite display the commanding presence I might have hoped for. Endrik Wottrich, as Florestan, has a humdinger of an entry: at the beginning of Act II, as the curtain rises on his dismal cell, his anguished exclamation "Gott! Welch Dunkel hier!" (God, how dark it is here) was splendidly dramatic, and he carried off the (very difficult) ensuing aria with aplomb. We had a nice cameo from Willard White as the deus-ex-machina Don Fernando; the only real disappointment was John Wegner, whose Don Pizarro was a bit underpowered and not really evil enough. It's not a subtle role - more or less a caricature of a villain - and Wegner came across as suave and urbane rather than actually violent.

The direction of the opera didn't really inject much drama and pace in Act I: that was left to the music. The acting wasn't always helped by staging and costumes that were fairly neutral. A peaked cap made Stemme suitably masculine but obscured her facial expression, and since the stage was almost totally dark for Florestan's cell, we couldn't see Wottrich's face at all. The long ladder leading down to the cell from the outside world worked well at providing the atomosphere of isolation, although the fact that the cell took up the whole stage made it lack the required claustrophobia.

Last night's production made the best of Fidelio's virtues, while failing to transcend the work's difficulties. Fidelio isn't the most effectively dramatic of operas, but it's a wonderful piece of music and worth seeing for that quartet alone, particularly when it's done quite as beautifully as last night.