A Druid chief-priestess sworn to chastity, a torrid carry-on with an occupying Roman general, illicit children (and attempted infanticide), a love-tussle with a junior priestess, a Roman intruder discovered “in the virgin novices’ cloister” … Norma (to 8 October) has a plot that can raise a wintry smile.

But Vincenzo Bellini’s 1831 opera, where poised neoclassicism meets passionate Romanticism, is no more stupid a drama than the Medea and Phèdre which inspired the French play by Alexandre Soumet on which it is based. And Bellini used it as a vehicle for something entirely new in music and opera, while creating one of the most idolised of starring roles. Norma – the glorious vehicle for the voices of Sutherland and Callas – is an incredibly demanding part, and Covent Garden had a serious problem when the star, Anna Netrebko, pulled out in April; recasting at such short notice was a chilling prospect. The young Bulgarian soprano Sonya Yoncheva stepped in to make her role debut, which she pulls off with considerable guts and style.

The Bellini wizardry looks simple (but isn’t). Over the most apparently naive accompaniment he unspools slow tunes of immense arching beauty; the composer aimed to make people “weep, shudder, die” through pure melody – nor is this an extravagant claim. His signature tune is “Casta diva”, Norma’s opening address to the silvery moon. If the ethereal notes work, the audience is entranced. Yoncheva did a very decent job of it, but was more at home in the heroine’s fiery and anguished music than these lyrical rhapsodies.