MONSTER-IN-LAW

(two and one half stars)

Fonda rules!

Running time: 95 minutes. Rated PG-13 (sex references, moderate profanity). At the Empire, the Lincoln Square, the Cinema 1, others.

WHAT becomes a legend most?

For Jane Fonda, it turns out to be the rote romantic farce “Monster-in-Law” – a cannily selected vehicle that’s more than funny enough to relaunch her as a major movie star after a 15-year layoff.

Fonda is a hoot and a half as Viola, a Barbara Walters-esque TV diva who schemes to separate her son from his fiancée, Charlie (Jennifer Lopez in her first relaxed performance since “Out of Sight”).

Then again, the much-divorced, hard-drinking Viola was having a hard time even before she met Charlie.

She went berserk on the air while interviewing a Britney Spears/Paris Hilton-style airhead who thought Roe v. Wade was a boxing match.

Viola is barely out of a lengthy stint in rehab when her beloved only son, Kevin, a hunky doctor (Michael Vartan), turns up with Charlie – and obliviously proposes in front of his still shaky mother.

Viola professes to welcome Charlie to the family, but her mind is clearly working overtime to eliminate the interloper, a free-spirited dogwalker whom Viola considers unworthy of her son.

As played by the bland TV actor Vartan, Kevin is pretty much treated as a plot device, sent off-screen to a medical convention until the climax while the female stars go head to head.

With the contrivance of her incredulous, long-suffering assistant, Ruby (the scene-stealing Wanda Sykes), Viola feigns insanity and moves in with her prospective daughter-in-law – and proceeds to try to drive Charlie crazy.

Merrily sending up Fonda’s own image (and reportedly that of her ex-spouse, Ted Turner), Viola throws an engagement party at which she not only name-drops mercilessly, but connives to force Charlie to squeeze her bountiful booty into a too-tight gown.

Charlie, who comes equipped with the inevitable gay pal (Adam Scott), finally has had enough – and Anya Kotchoff’s script builds to a bitch-slapping finale that’s as hilarious as it is predictable.

Director Robert Luketic (“Legally Blonde” and the risible “Win a Date With Tad Hamilton!”) keeps things moving swiftly enough that audiences won’t dwell on the many improbabilities.

Most important, he lets his 67-year-old icon deliver a fierce, vanity-free performance that reminds us why she’s won five Oscars and was nominated for five more.

While Lopez is never photographed in a less-than-flattering manner – Charlie even wakes up in full makeup – the stiffness and tentativeness that’s afflicted her recent work is gone, perhaps due to Fonda’s inspiration.

“Monster-in-Law” is a triumphant comeback for Fonda, whose acting chops complement a presence that puts most of today’s stars to shame.