Thomas Adès, London Symphony Orchestra, Barbican Hall

Strindberg provides the libretto, an ingenious little character sketch in which two rival actresses (Madame X and Mademoiselle Y) catch each other by chance in a café. Only Madame X does the talking. She trundles through various levels of small talk, working herself up - and down - realising that she is in fact face-to-face with her husband's mistress. The title, La plus forte, "The Stronger", reflects Madame X's final thought that, though Mademoiselle Y may have had her husband, and loved her husband once, she, Madame X, will have him and love him forever - or so she hopes.

Barbara Hannigan is mesmerising as the garrulous Madame, who saps her own strength by talking and talking and talking, incriminating her damaged self through every one of her transparently spiteful sentences. It's a part that could be played so many different ways. Hannigan delivers it without coarseness or caricature, pitching it on the level. The grip of an absorbing reality, of an overheard top-of-the-bus encounter, is at the heart of Hannigan's appeal. Psychology - pure, still, stationary - is all there is here. And it makes for a thrilling theatrical experience.

The music does everything to heighten the tensions between Madame X's attempts to maintain the mundanity of the conversation and her failure to keep meaning from seeping out from every innocuous pore, Barry's score consistently betraying the insecurities behind her irritations at, for example, the servants not being able to make a decent cup of coffee. Sentences get stuck in rhythmic and melodic grooves, like the speech-patterns of teenagers - grooves that her nerves clasp onto in fear. These are expanded upon in the orchestra, naturally, intuitively, almost programmatically. They are mugged by the LSO brass or filched by strings in pizzicato, always maintaining a basic shape, developing in pitch. Two thirds of the way in, an atmospheric return to diatonicism emerges that ushers in a unison staccato tread that the orchestra takes up to the end. Throughout, there is the feeling that one is clinging onto the heartbeat of Madame X as she tries to keep a check on her subconscious wilds.

The Thomas Adès that filled much of the rest of the programme was interesting - and perhaps would have been even more interesting if La Plus Forte hadn't been there to trump it. ... but all shall be well (1993) is a very early, transparent work, with an attractive exposition. The melody, however, doesn't hold up under the weight of development and recapitulation. These Premises Are Alarmed (1996) is set up as a bon bon in the notes but in fact revels in a climax of searing romanticism, which disintegrates and dissipates amid untuned percussion. This device of emerging and returning to the untuned is explored in Powder Her Face. The dances from this opera rounded off the evening in a dark, drug-induced fashion, Adès pushing things to their brash, noisy, sluttish, logical conclusion.

The highlight should have been the Bartók Piano Concerto No 1 (1926) with Zoltán Kocsis, that lionised Bartókian, at the keyboard. But things between conductor and soloist never quite clicked. Kocsis kept failing to project; Adès kept failing to keep control and the result was a disjointed and hairy ride through this mildly baffling Modernist-primitivist romp. In the end, the title said it all: La plus forte was, indeed, la plus forte. Barry's work is a game-changer. 21st-century opera begins here.