Twins Viola (Imogen Stubbs) and Sebastian (Steven Mackintosh) wash ashore on a foreign land, and are separated. Viola disguises herself as a man for protection, and goes to work for Duke Orsino (Toby Stephens) as his page.
Orsino is in love with Olivia (Helena Bonham Carter) and sends Viola to her to plead his case. Viola ( AKA Cesario) pleads the case so eloquently that Olivia falls in love with her/him.
Later, Olivia meets with Sebastian, believes him to be Viola/Cesario, and marries him. Viola then reveals herself to Count Orsino, becoming his lady.
(Note: The movie is good, but the best reason to see it are the subcharacters: Ben Kingsley as Feste the Fool and Nigel Hawthorne as Malvolio)
Thanks Paul B!