And one, and two, and... No, toes turned in!

Ballet audiences aren't normally given to dishing out verbal abuse. But on the night of 29 May, 1913, the Theatre du Champs Elysees in Paris famously erupted in cat-calls and jeers. The occasion was the first night of Vaslav Nijinsky's two-act ballet Le Sacre du Printemps (The Rite of Spring), with music by Igor Stravinsky. The work ran for five more performances, then sank without trace. Stravinsky's phenomenal score went on to have a vibrant life in the concert hall. Nijinsky's mould-breaking choreography was - without apology - forgotten.

Until, that is, more than half a century later a pair of eager-beaver historians named Millicent Hodson and Kenneth Archer set about piecing it back together, principally from written descriptions by Nijinsky's sister, Bronislava, the living memory of the 90-year-old Marie Rambert who had been Nijinsky's assistant, and scrupulously informed guesswork. Their research took 17 years and spanned five countries in three continents, and the result - premiered by Chicago's Joffrey Ballet in 1987 - has since been mounted by Hodson and Archer on eight different ballet companies around the world.

Yet it has taken until now - 90 years after the original premiere - to restore the ballet to its spiritual home: the Kirov Ballet of St Petersburg, Alma Mater not only to the ill-fated Nijinsky, who was confined in a lunatic asylum in 1919, but to almost all the dancers of the Ballets Russes. A still more extraordinary fact is that until a couple of months ago, today's Russian dancers had little idea of their lost heritage.

"You mention Nijinsky to any other ballet company," says Archer, "and the dancers regard him as their patron saint. In St Petersburg, they think of him [as] merely a famous ballerina's partner. He and Diaghilev had committed the crime of taking their talent to the West and not coming back, and the Soviets virtually wiped them from history. We've had to re-educate the dancers in their own culture." But the Russians haven't made it easy for them. When Hodson and Archer arrived in St Petersburg earlier this year, armed with their bulging notebooks and diagrams to teach the ballet to the company, the effort and stress of the process very nearly broke them.

"The Kirov dancers lead a very fragmented life," explains Hodson, "touring constantly, and rehearsing and performing things they've been doing at the Maryinsky Theatre for 120 years, which means that new work, even new-old work, is unusually taxing for them." Rehearsals were not helped by the fact that Rite requires 47 dancers, the company needed two casts, yet touring commitments meant that substitute dancers continually turned up instead. "The Kirov has 260 dancers in all," sighs Hodson, "and I must have ended up single-handedly teaching the steps to 130 of them." Demonstrating what Nijinsky wanted (or what Hodson believes he wanted) was one thing. Persuading the dancers to take it on board was quite another. Hodson describes the movement in Rite as "very hard and carved and angular", with the toes permanently turned in, the very opposite of the turned-out classical ballet stance dancers are trained in all their lives. "Plus, on principle, Nijinsky abandoned all the French terms commonly used in ballet. He favoured graphic terms such as 'zigzag jump', 'hands on the seams of the trousers', 'squat' and 'fling'. He really felt that new dance required new language." To make things more difficult still, Nijinsky seems to have been determined to give bodily expression to every last polyrhythm of Stravinsky's mathematically fiendish score. At one point there are three time-signatures going on. To give encouragement, Hodson kept reminding the Kirov dancers that the piece was made on Maryinsky dancers in the first place. She didn't dare tell them that the original cast revolted.

The breakthrough came, says Archer, with the men. "They liked the intellectual challenge of all the counting. And it's tough. They have to do one count for the lower half of the body and a different count for the upper half. It's like patting the head and rubbing the stomach. They also seized on the virility of the movements. And once their girlfriends started coming in to watch, and saw how vigorous they looked, they began to get competitive." Another watershed point came when the rehearsal pianist was playing through a section of the score and suddenly recognised it as something from Russian folklore. She started singing: a ritual call-and-response chant she'd learnt in the playground as a child. All of a sudden, says Archer, the music had an intimate connection with these people, it had meaning.

When the Kirov bring The Rite of Spring next month as part of their London season, Hodson and Archer will be secure in the knowledge that their vast, historic undertaking is achievable: at the St Petersburg premiere last month it was touch-and-go until the 11th hour, with no proper technical run and a question-mark still hanging over the conductor's tempi.

Hodson and Archer had been "plotting for months" how best to tackle this last delicate problem. Ballet music is invariably taken at a faster lick once it has a life outside the theatre (think of Tchaikovsky's Nutcracker Suite), and Rite is no exception. This was the couple's big worry, not least since Valery Gergiev's recording of the work ("which he claims is exactly as Stravinsky intended") differs significantly from the Pierre Boulez recording ("which Boulez claims is exactly as Stravinsky intended"). Hodson and Archer had tried using Gergiev's recording in rehearsals, "and always at the same place the dancers would come to grief - it was just too fast for the choreography." Eventually their chief rehearsal pianist - the one who had sung the folk song - managed to drop a tactful word in the maestro's ear. At the premiere, no one fell over.

Given the extent to which Rite of Spring has dominated Hodson and Archer's professional and personal lives (they married in the course of their research), you wonder how they feel about the countless other interpretations in circulation, by artists ranging from Maurice Bejart to Walt Disney to Pina Bausch. Kenneth MacMillan's version, made in 1962, is being revived by English National Ballet in London this week.

As it happens, they both "loved MacMillan's version even before we met, and for the same reason: because it's based on a physical and spiritual ordeal. The whole idea of Nijinsky's Rite is that every dancer on stage goes through this primordial ritual process, and in MacMillan's you feel they do. The focus of the ritual is the sky, the cosmos, but the dancers are a tribe, trying to achieve a single result, the banishment of winter and the return of spring. It's this ritual element, more than anything else, that made the ballet revolutionary.

"In truth I don't feel we have the right to disapprove of anyone's version. Because we've tried to go back to the original doesn't invalidate anything that followed. If anything, it gives a basis for validation. Once the legend is no longer complete legend, once there's an artefact, then we have a treasure in our collective consciousness that allows us to react." For most of us, reactions will range between curiosity and scepticism. Whatever the degree of authenticity in the Hodson/ Archer reconstruction, what's certain is that the effect of the 1913 Le Sacre du Printemps can never be reproduced.

ENB's 'Rite of Spring': Sadler's Wells, London EC1 (020 7863 8000), Tue & Wed. Kirov season: Royal Opera House, London WC2 (020 7304 4000), 4 & 5 Aug