The film begins with a chase through the back streets of London set to jaunty piano. Then, after brief stops at Algernon Moncrieff’s town house and Jack Worthing’s country estate, it bustles off to a smoky dancehall where floozy-looking women waggle bloomered bottoms at swells in scarves. So far, not a cucumber sandwich in sight. But soon that perfect gem of the Victorian stage, Oscar Wilde’s The Importance of Being Earnest, will swing into view, albeit with such additions as ballooning, boating, tattooing, and a fantasized Algie in armor. Algie and Jack will meet at the dancehall and retire to the former’s digs, where they will eat the cucumber sandwiches handsomely laid out by Lane the butler for Algie’s Aunt Augusta, the formidable Lady Bracknell. And when Lady B. asks for the dainties, Lane, without a hint that his employer has gobbled them, will assure the company that there were no cucumbers at the market, "not even for ready money."

British adapter/director Oliver Parker, whose most recent gambit was a film version of Wilde’s An Ideal Husband, opens up the author’s famously coined 1895 "trivial comedy for serious people" in ways that might seem sacrilege to the perpetrators of the stagy 1952 Anthony Asquith film. Yet even while engineering us about various settings in teeming town and idyllic country awash in sunshine and birdsong, Parker maintains most of Wilde’s wittiest dialogue, as well as the perhaps not entirely frivolous plot involving the secret lives of Victorian gents.

Sober country squire Jack has invented a profligate brother, Ernest, whose identity he assumes in London. Algie has fabricated a very ill friend called Bunbury to whose bedside he must hasten when he wants to escape his social obligations. As Ernest, Jack proposes to Algie’s cousin, the willful and romantic Gwendolen Fairfax, who is determined to wed a man called Ernest. Lady Bracknell, however, finds Jack’s foundling origins as unacceptable as Gwendolen would his real name. (For him to have lost one parent, the mother famously opines, might be counted a misfortune; to have lost both "seems like carelessness.") Meanwhile, as Jack frantically sets about addressing Gwendolen’s authoritarian progenitor’s concerns, Algie appropriates the wicked Ernest to romance Jack’s tasty 18-year-old ward, Cecily Cardew, who also has a predilection for men called Ernest — and, in the film, a positively Arthurian imagination.

In addition to imbuing Earnest with madcap and fantasy elements (not to mention an incident borrowed from a little-known four-act version of the play), Parker has assembled a delectable cast led by Judi Dench as the most adorable dragon since The Land Before Time. Rupert Everett is a slightly smarmy Algie who, when he takes over Ernest, becomes slyly doe-eyed — though no less mischievous. Colin Firth’s Jack is a relative straight man, but Parker has invented a sort of raucous sibling relationship for the two, even before they know they’re siblings. Whether Jack is strangling Algie or the pair are serenading their frosty girlfriends (with "Lady, Come Down," lyrics by Wilde) as servants lug a grand piano about, contemporary testosterone gets poured on Victorian manners.

Frances O’Connor is a dreamy, sensuous Gwendolen, albeit with seeds of her mother. And Reese Witherspoon, the only American in the cast, is adorable enough to bring off the film’s most questionable addition: Cecily’s daydreams of being tied to trees and romanced by a knight in armor, complete with orchestra in the bushes. As her reedy tutor, Miss Prism, Anna Massey ably suggests a woman only loosely tethered to her asceticism. And Tom Wilkinson, as her admirer, the Reverend Dr. Chasuble (here a closet painter whose briefly glimpsed works are Pre-Raphaelite riffs on Miss Prism), is an almost robust booby. (These two parts have been beefed up, allowing Wilkinson’s Chasuble, beating a retreat, the best line not Wilde’s, that he has a double baptism and needs "to top off the font.")

Dench, though in perfect command of Lady Bracknell’s seemingly thesaurus-derived pronouncements, is no gorgon. Rather, she makes use of her diminutive stature, rising imperiously to her full five feet when dismissing the suitor whose closest relation is a "parcel," then suggesting her own pent-up nature by tearing his application into Enron-worthy shreds. Every now and again, a smile plays around her lips, as if waiting to explode into full-blown laughter at the end. And though Witherspoon has the prettiest costumes, Dench’s are the funniest, with feathers and then flowers exploding from the shoulders of her trussed-up form.

Parker supplies some wonderful, wordless moments, among them a shot of Gwendolen and Cecily reading each other’s diaries for recreation. There are a few puzzlements, some in the form of fidelity — Gwendolen retains Wilde’s line that Mr. Worthing "asked me to be his wife yesterday afternoon at 5:30" when the film has shoehorned in way too much incident for that to be true. But by and large, this sumptuous lark of an Earnest goes toward proving its own assertion that "in matters of grave importance" — say, the adaptation of a masterpiece — "style, not sincerity, is the vital thing."