It's 1753 Venice and Giovanni Giacomo Casanova (HEATH LEDGER) is the locally born adventurer with a famous reputation as a ladies man. His latest such conquest -- of a young nun -- has him in hot waters with Vatican authorities. That leaves his local protector, the Doge (TIM McINNERNY), with only one solution and that's either settle down and get married or face the consequences.
Accordingly, Casanova and his assistant Lupo (OMID DJALILI) set out to find a suitable mate for him and believe they've discovered that in the lovely virgin Victoria (NATALIE DORMER). Her father couldn't be any more pleased to give his daughter away, but her neighbor across the street, Giovannni Bruni (CHARLIE COX) isn't as happy. And that's because he's madly in love with Victoria, but has never acted upon it.
His sister, the outspoken and pre-modern feminist Francesca (SIENNA MILLER), is in her own romantic predicament. It seems her late father pre-arranged a marriage for her, the thought of which she abhors. But since her mother Andrea (LENA OLIN) doesn't have the money to support them, Francesca is resigned to the thought of a loveless marriage.
Things become more complicated when Giovanni confronts Casanova about stealing away his girl, thus prompting Francesca to stand in for him during the ensuing duel. When Casanova learns it's really her, however, he's instantly smitten, but lies about his identity since she loathes the thought of his real self and legendary status.
Hoping to win her over, Casanova intercepts her suitor, the obese but rich businessman Paprizzio (OLIVER PLATT) who's led to believe - by Casanova and Lupo -- that Casanova is really a renegade author who's enthralled the public but angered the political types with his controversial writings. That writer as well as Casanova's reputation has summoned Bishop Pucci (JEREMY IRONS) from the Vatican who's determined to squash both heretics. From that point on, and as the ruse gets more complicated, Casanova does what he can to win over Francesca, the only woman ever to have won over his heart but who's also the least likely to succumb to his innate charms.