Prom 42: BBC Symphony Orchestra/Gergiev, Royal Albert Hall, London

You may think you know St John's Night on the Bare Mountain - to give Mussorgsky's satanic potboiler its full and proper name - but a few bars into the piece that opened this all-Russian, all-pagan, all-singing/dancing Prom, that which is so familiar is tossed back into the cauldron and comes out sounding, well, bewitched.

You may think you know St John's Night on the Bare Mountain - to give Mussorgsky's satanic potboiler its full and proper name - but a few bars into the piece that opened this all-Russian, all-pagan, all-singing/dancing Prom, that which is so familiar is tossed back into the cauldron and comes out sounding, well, bewitched.

Mussorgsky revised his dramatic tone-poem twice, but it was Rimsky-Korsakov who turned it into the classical favourite we know. Mussorgsky's rarely-heard original (from 1867) is another matter, another piece.

The shock tactics begin from bar one. No eerie crescendo to mark the approach of those malevolent spirits: the string flurries arrive in howling fortissimo, the witches flying in chaotic formation. Shrieking piccolos, grunting basses, a moment of madness for bass drum straight out of Stravinsky's Rite of Spring, and we're right at the heart of the commotion. Mussorgsky's "scattered variations" on a folkish theme (how he described the piece) are stark and disorderly, the more so when conducted by the volatile Valery Gergiev, who seemed to turn every change of metre into a potential crisis. The BBC Symphony Orchestra hung on to his somewhat erratic beat. Just. It didn't sound entirely secure, but is it supposed to? Rimsky-Korsakov gave the material coherence, but neutered its hocus-pocus in the process.

Mussorgsky was to have supplied a black sabbath sequence for an opera-ballet, Mlada, to be co-composed by four of Russia's leaders in the field, but it never happened. Rimsky-Korsakov's version did, though hearing the fragmented hotchpotch of its third act, the effect is still one of unfinished business.

Of course, there are wondrous orchestral effects: the shades of night in cascading violins at the outset (the Mantovani effect); the heady arabesques of duetting clarinets and the fabulously ornate invocation of Cleopatra deep in the strings' chest register; the intimations of Wagner, with one idea quite spookily close to the "tarnhelm" motif from the Ring. The fruity mezzo of Olga Savona and high-pitched tenor of Avgust Amonov, somewhat anxious in his ecstasy, contributed almost incidentally; the Apollo Voices despatched strident folksiness. But it just didn't add up.

Then, we heard where all this was leading. Gergiev's account of The Rite of Spring made clearer than ever the Rimsky connection: the very fluid, lyric cast of his diaphanous introduction with those very Eastern arabesques. They might have been Rimsky, a radical reinvention of Sheherazade. And the pagan plainchants - like very old voices. The songful and folkloric were once again the prime movers in this reading: those, and the extremes of barbarism and beauty.

Gergiev's reckless speeds and potent pauses made for real edge-of-the-seat tension and atmosphere. By today's standards it was far from perfect, but it was spontaneous and unpredictable in ways that performances of this piece rarely are. Gergiev truly turned back the clock. And in the gaping silence he opened up before the final spasm, he shocked once more.

Booking: 020-7589 8212; www.bbc.co.uk/proms. Prom 42 available online to Monday