When does a secret become a lie? Opera Holland Park’s double bill gave two contrasting answers. The first, from Ermanno Wolf-Ferrari, was frivolous, an airy vol-au-vent to be paired with champagne on a summer evening. The second, from Tchaikovsky, was in complete earnest, with music as transportingly romantic as you’ll hear in any opera and three individual performances out of the very top drawer.

You have to take Il segreto di Susanna for the period piece it is – Wolf-Ferrari's opera dates from 1909. Susanna’s guilty secret, which she is convinced will be ruinous to her recent marriage to Count Gil, is that she is a smoker: the gag is that her jealous but sharp-nosed husband is so incapable of imagining that a woman might smoke that he is certain that she has taken a lover. All becomes clear in the course of an hour of amusement, acted with willing enthusiasm by Clare Presland and Richard Burkhard, with the straight-man help of John Savournin’s non-speaking butler. Presland plays up the glamour and sings with a mezzo voice that’s especially creamy and attractive in its middle register; Burkhard’s baritone is mellifluous and he somehow manages to avoid looking ridiculous in a pink suit that would sink a less confident performer. Conductor John Andrews keeps the music tripping merrily along and director John Wilkie ensures that there’s plenty of movement around the stage and enough visual gags to hold our interest. This isn’t an opera that will stay long in the memory, but it made for fine entertainment on a balmy summer evening.

Tchaikovsky’s Iolanta is altogether more substantial fare, with a far darker secret: our princess heroine has been blind from birth, but King René, unable to bear the thought of telling his adored daughter what she is missing out on, has walled her up in a lovely garden, surrounded by servants who engage in fearful mental gymnastics to avoid mentioning anything about light, colour or beauty. No plan, however, survives its first contact with the enemy: René’s is torn to shreds with the arrival of Robert, Duke of Burgundy (to whom Iolanta is promised in marriage) and Robert's friend Vaudémont (who immediately falls in love with her). Amor vincit omnia, of course, and after Vaudémont and Iolanta have negotiated various rocks in the path of true love, they all live happily ever after.

It’s utter hokum, but Iolanta is the ultimate demonstration that great music can trump just about anything. What Tchaikovsky produces is so earnest, so yearning, such a direct injection of the fairy tales of one’s childhood that you cannot fail to be seduced. After an uncertain start with some decidedly iffy woodwind intonation, Sian Edwards and the City of London Sinfonia played it to perfection, with complete mastery of the ebb and flow of the long melodic lines, immaculate balance of voices against orchestra, of woodwind quotes against string swell, with horns invoking ancient tales of fair ladies and knights errant. It all added up to the most delicately weighted outpouring of pure romanticism.

Blended into this orchestral masterclass were two fabulous voices. Natalya Romaniw’s phrasing was wonderfully in sympathy both with the orchestral swell and the simple innocence of her character: her voice is so smooth and rich across the range as to be utterly bewitching. Butt Philip’s voice has warmth and earnestness to match and he judged the pace of the work well – almost eclipsed by Grant Doyle’s Robert at their entrance, then gradually ratcheting up the intensity. His duets with Romaniw were swoon-inducing, particularly at the moment when he realises that she is blind. Good support was provided by Doyle’s noble baritone and the rich bass of Mikhail Svetlov’s King René.

Olivia Fuchs’ direction didn’t convince, either in setting or in acting. I’m not sure what prompted the somewhat drab setting in a 1940s hospital, with Iolanta drugged to sleep by René’s servants, nor what purpose was served by having the two lovers in decidedly frumpy costumes – she in white hospital nightdress, he in decidedly bucolic clothing. Stage movement was less than crisp: the body language of the principals failed to live up to the wonderful sound they were producing.

An evening of contrasts, therefore: Wolf-Ferrari’s music is pleasant enough but little more, but some taut acting and directing made Il segreto di Susanna into an entertaining piece of drama. Iolanta may not have worked as a piece of theatre, but the glory of Tchaikovsky’s music sent us into the night with our hearts uplifted.