Early on in the new film “Ballets Russes,” narrator Marian Seldes tells us that the documentary will recount “the story of the birth of modern ballet.” That isn’t exactly true.

The story of the birth of modern ballet would be the story of the original Ballets Russes, formed by the great impresario Sergei Diaghilev in 1909. Diaghilev used dancers from the Imperial Mariinsky Theater in St. Petersburg, and his company performed in Europe when the Mariinsky was on vacation. In place of the full-length spectaculars of the 19th-century repertory, Diaghilev offered a new repertory of one-act ballets, as well as a movement style that broke away from the old classical technique. After a rupture with the Mariinsky in 1911, Diaghilev’s troupe dominated European ballet until he died in 1929.

“Ballets Russes,” directed by Daniel Geller and Dayna Goldfine, instead concerns the companies formed during the 1930s to fill the void left by Diaghilev’s death. The new Ballets Russes starred several Diaghilev alumni, along with a fresh crop of emigre dancers – some Russian, some European with Russified stage names. They perpetuated the formula Diaghilev established, performing many superlative works and showcasing much magnificent performing.

Priceless performance footage, lovingly vetted and prepared, makes this film riveting: It channels an extraordinarily vibrant performance style that has been lost. A few seconds of George Zoritch’s “Afternoon of a Faun” or Tatiana Riabouchinska and Nathalie Krassovska’s Prelude from “Les Sylphides” are more revelatory than many complete performances of these works today.

We also see the cognates of today’s style. Devoid of explict plot, Leonide Massine’s “symphonic ballets” instead highlighted abstractions such as Alicia Markova’s poetically distilled tragedy in “Rouge et Noir.” Other dancers strive for the same movement principles as today: Irina Baronova’s supporting leg straightens tautly when she rises on pointe in the grand pas de deux from “Sleeping Beauty.”

The film’s organizing principle is anecdotal. It weaves personal narratives of dancers – some legendary, some members of the corps de ballet – with the history of the companies, which splintered several times during the 1930s. Not without a taste for sentimentality, the filmmakers catch up with the dancers in their old age and follow them as they continue to work out at the barre or in the gym, or coach young dancers in the roles they danced decades earlier.

We learn, for instance, that the unwitting architects of the Ballets Russes were the mothers, many of them refugees from the Russian Revolution, who took their daughters’ destinies into their hands and sent them to the Paris studios of ex-Mariinsky ballerinas Lubov Egorova, Mathilde Kchessinskaya, and Olga Preobrajenskaya, each of whom had also fled Revolutionary Petersburg. Baronova recalls that at first she wasn’t keen to study ballet, but “my mother took me by the scruff of my neck. … One didn’t argue with my mother.”

The mothers’ ambition and economic desperation helped to fan an ensemble of extreme precocity. Mia Slavenska recalls that she was on pointe at age 4, which today would be considered an act of reckless abuse. Her mother “just cut the front of my baby shoes out and just made a slipper, like any kind of slipper, and I was on toe.” These were, after all, the years of Shirley Temple and Judy Garland; the vogue was for child stars in every genre of entertainment. But inclusion in the Ballets Russes subjected teenagers to a grueling regime, and it certainly took its toll. Several of the troupe’s greatest stars retired early.

In part because of its dependence on interviews, “Ballets Russes” will not please scholars. The films’ creators were evidently charmed, perhaps too charmed, by their subjects – and indeed, these ex-performers retain an undimmed sparkle. At several junctures, the directors seem not to know enough about historical events to place in context recollections that seem self-serving, and they are reticent to dig deeply – even posthumously – into the psychologies of the leading lights (although we do hear in passing about Massine’s greed and Balanchine’s need for total control).Worse, much of the footage is never identified, even that showing some of the most important performances from the Ballets Russes’ legacy. Nor are the silent films cued to the exact music they were filmed to.

The film spends a great deal of screen time, perhaps too much, on the declining years of the Ballets Russes before its dissolution in 1962. The troupe grew artistically moribund, forever on tour, starved of new works. Its programming of mainly one-act narrative ballets fell out of favor, capitulating to Balanchine’s postwar template of pared-down decor and costumes, and complete liberation from plot.

During the last 20 years, the Ballets Russes repertory has enjoyed an international revival. There are glaring differences between their ballet and ours, however. Legs today are much lighter, higher, freer. Bodies are sleeker. Turnout is better, pirouettes more graceful, the approach to technical stunts more suave. But today’s insistence on high legs has diminished the expressivity of the torso and thus cut off ballet from an essential engine of communication.

What is most startling about the Ballets Russes performers is their lack of self-consciousness when required to express any emotion, participate in any dramatic interaction. Their personalities surged across the footlights, untrammeled and uninhibited, and this above all is why they inspired such adoration.

“Ballets Russes” plays at Film Forum until November 8 (209 W. Houston Street, between Sixth Avenue and Varick Street, 212-727-8110).