Heath Ledger plays the title character in "Casanova." Click photo for larger image. 'Casanova' Rating: R for some sexual content Starring: Heath Ledger, Sienna Miller, Jeremy Irons, Oliver Platt, Lena Olin Director: Lasse Hallstrom 'Casanova' Web site

The real Giacomo Casanova (1725-98) -- adventurer, spy and sensualist intriguer -- was fierce, but this film is farce from its frantic opening: Heath Ledger runs amok through a Venice convent, violating more than just the novices' vows, with an Inquisition SWAT team in hot pursuit.

"A night with Casanova means eternal damnation!" the nuns are warned. "Seems fair," one replies.

The title character of this latest "Casanova" is soon nabbed and sentenced to death, but his punishment is reduced from capital to marital: He must find a wife and settle down -- a fate arguably worse than death but surely not difficult. The great libertine lover can get any woman he wants, after all, and all women want him.

Except Francesca Bruni (Sienna Miller), a proto-feminist on the cutting edge of women's lit & lib. She's the secret author of "The Hopes of Women in a World of Men," the runaway best seller of 1753, and crashes the all-male university debate on that subject, disguised as a man.

This drag act could fool only the Venetian blind. But never mind. She delivers an impassioned Gloria in Excelsis Steinem speech that fuels the absurdly anachronistic dialogue. "What you imagine to be love is in fact self-love," she instructs Casanova.

Instruction in seduction, on the other hand, is his game, with Francesca's fat fiance (Oliver Platt), who seeks the surefire way to her heart.

With all its duels and disguises, masked balls and mistaken identities, slapstick and chases galore, "Casanova" aims to be a sophisticated comic-romantic romp, but any attempted channeling of "Tom Jones" or "Shakespeare in Love" fails.

The real Caz was jailed as a magician in Venice, escaped, became director of lotteries in Paris, where he amassed a fortune, served as a double or triple agent of Louis XV and was later exiled for "satirical libel" of a patron. Mozart consulted him before immortalizing his own favorite serial seducer in "Don Giovanni."

Sienna Miller, left, plays Francesca Bruni, a Venetian feminist who is pursued by the title character Casanova, played by Heath Ledger, right. Click photo for larger image.

With such rich REAL material, why bother with fake silliness?

Well, 50-some films, including the great Fellini's, have faked it before. But this rake's progress is impeded by the worst script (with the least ironic sophistication) since "Police Academy IV."

Ledger, recently descended from "Brokeback Mountain," is overexposed and underdeveloped. Miller is mildly charming. Platt is totally sick-making. Jeremy Irons is funny as the Inquisitor, kvetching that "the people here are given over to physical pleasure and skepticism."

"Casanova" is rated R for "some sexual content," but I dare you to find it.

I'm a big fan of director Lasse Hallstrom, whose films in general -- fine "Cider House Rules," "My Life as a Dog" and "What's Eating Gilbert Grape?" in particular -- are distinguished by their sensitive concern for human nature. But "humanizing" Casanova by turning him into a monogamist is like making Faust marry Gretchen and adopt an orphan at the end. Bawdy French farce and Italian opera buffa don't come naturally to serious Swedes like Hallstrom.

Lasse, come home!