Reserved bachelor Jack Worthing (Colin Firth) enjoys a simple, responsible life with Cecily Cardew (Reese Witherspoon), his utterly romantic but romantically sheltered niece. Their life in the country is quiet and serene, save for the moments when Jack is whisked away to London to fix the problems caused by Ernest Worthing, his wayward and carefree brother.
Trouble is, Jack is Ernest. As an escape from the mundane, Jack assumes the identity of this roguish, imaginary alter-ego. Goodbye simple and responsible, hello wayward and carefree. Once he's in London, Jack teams up with his ne'er-do-well partner-in-crime, Algy Moncreef (Rupert Everett), a man whose penchant for overspending is matched only by his knack for dodging bill-collectors.
Jack is also in love. Well, Ernest is, at least. The lucky object of his affection is Gwendolen Fairfax (Frances O'Connor), a rebellious aristocrat with a flair for the dramatic. Jack - as Ernest - proposes to the smitten Gwendolen. Convinced that she is destined to marry a man named Ernest, she graciously accepts his offer. But there's a hitch: Gwendolen's mother, the formidable Lady Bracknell (Dame Judi Dench) requires more than just a name from her daughter's suitors. When she interrogates Jack about his social credentials, she learns of his inauspicious beginnings and refuses her consent. Thus, in order to wed Gwendolen, Jack must rid himself of Ernest and find some evidence of a worthy ancestry.
Algy, meanwhile, takes advantage of his friend's preoccupied state by paying a visit to Cecily. When he introduces himself as Ernest, he is delighted to discover that she has long dreamed of marrying the errant brother.
When Jack returns from the city with news of poor Ernest's death, he is shocked to discover that the figment of his imagination has taken on flesh and blood. Further exacerbating the situation is the arrival of Gwendolen - looking for her Ernest - a debt collector wishing to escort Mr. Ernest Worthing to prison, and a furious Lady Bracknell in search of her elusive daughter.
But waiting around the corner is a surprise - one that affects each and every one of them and will reveal, once and for all, The Importance of Being Earnest.