The big question isn't how Lone Scherfig's "An Education" became one of the word-of-mouth sensations of this year's Sundance Film Festival or how it spawned a bidding war won by Sony Pictures Classics.

No, the question is how a film this assured, this primed for critical and popular acceptance was able to arrive under the radar at all.

Much of the buzz surrounding "An Education" has surrounded Carey Mulligan's star-is-born performance. She's like Emily Mortimer crossed with Ellen Page crossed with Katie Holmes. She's emotionally open, fluid with both comedy or drama and she has a quirky beauty that plays differently no matter how you shoot her. She's the Belle of the Ball at this Festival and for good reason.

But only celebrating Mulligan probably sells short just how good "An Education" is. From Nick Hornby's script to the range of standout supporting performances to a soundtrack loaded with early rock hits and all tied together by Scherfig's direction, "An Education" is best of the 24 movies I saw at Sundance.

[More after the bump...]

Set in 1961, "An Education" is the story of a Jenny, 16-year-old trapped in a rut that will inevitably see her finish school, take her A-Levels and head off to Oxford, all without room for spontaneity or adventure. Elsewhere the '60s are beginning, but in her home there's only Latin homework. One day she meets David (Peter Sarsgaard), a much older man with money and the sort of life the French-speaking, art-loving Jenny dreams of.

I guess there's a version of "An Education" that might play out as a skeevy older man/younger woman Lolita-style drama, but Hornby's adaptation of Lynn Barber's memoir isn't even slightly salacious or exploitative. Perhaps because of the time period or English cultural conventions of the time, the age difference between the characters is mentioned a few times, but largely sans disgust or judgment. 

Prurience aside (and the movie is calculatedly tame about sexuality), there's some question of what Sarsgaard's David sees in Jenny, but the movie is more invested in her interest in him. She's investing all of her time and energy into getting into Oxford, aware that all of her efforts may not yield a job any more lucrative than teaching or social work. No matter how closely she follows her father's (Alfred Molina) instructions, the odds of her breaking through England's class system are low. She knows that, but she also sees how easily being with David allows her access to classical concerts, to fancy restaurants and to The Continent. The ideas about love and sexuality are there, but as her motives go, they're either secondary or they get mixed up with quality-of-life issues.

"An Education" is the rare period film that doesn't rely on over-obvious costuming and art direction or soundtrack or dialogue choices. Nothing screams "This is 1961 and The Revolution is Coming!" It's a huge relief. Instead, Hornby's concentration is on social dynamics and attitudes, how a middle class girl, even the brightest and most ambitious, would view life in 1961 England, how she'd approach wealth or religion or sex or the future. Hornby's script couches all of those things in dialogue that's clever and poignant without ever seeming overwritten. If Hornby isn't a top contender for next year's adapted screenplay Oscar, the fault lies with the Academy.

Director Scherfig did the Dogme thing on "Italian for Beginners," but here she lets the aesthetic of the film be guided by the evolution of her main character. The film seems to be more colorful and looser and more quickly edited as Jenny gets pulled into her new beau's world, more muted and constrained when she's in her old world. 

Mulligan is in nearly every scene of the movie, but there are at least a half-dozen well-developed supporting performances around her. Most poignant are Molina and Cara Seymour as Jenny's parents, who only want the best for their daughter, but have no idea what "the best" really means. Molina received a Tony nomination for a revival of "Fiddler on the Roof" and his performance is Tevye-esque, a father who recognizes that the world is changing, but can't bring himself to adapt. He mixes bluster with understated caring. Seymour's role has less dialogue and is therefore more inscrutable, but I was constantly drawn to her, just to gauge her reactions.

David offers the perfect character for Sarsgaard, who's usually hampered in his ability to play earnestness ("Shattered Glass" being an exception). Speaking with a British accent, Sarsgaard never seems completely trustworthy, teasing various aspects of the character for later. Dominic Cooper has now had two standout performances at this year's Festival, playing the most dynamic character in "Brief Interviews with Hideous Men" and here playing David's best friend. Also exceptional is Rosamund Pike, who never lets her bubbleheaded character, perhaps Jenny's exact opposite, seem like a caricature.

The cast also features Olivia Williams, Emma Thompson and recent Golden Globe winner Sally Hawkins in small roles.

How will "An Education" play away from Sundance? I wish I knew. Even with its all-star cast and top-notch pedigree, it's still a modest film, albeit a modest film delivered with intelligence and polish. Screening at 8:30 a.m. on my last day at Sundance, cold and damp from a mile walk in sleet, I was captivated from the opening credits on. I hope other people feel the same once Sony Pictures Classics releases it.