Ashton Kutcher, left, plays Jack Fuller, and Cameron Diaz is Joy McNally in the comedy What Happens in Vegas.

20th Century Fox / K.C. Bailey

Ashton Kutcher’s Jack Fuller is a life-of-the-party guy who gets fired from his job by his boss, who also happens to be his father.

Cameron Diaz’s Joy McNally is a go-getter Wall Street broker who has just been dumped by her boyfriend.

To forget their troubles, they head to Las Vegas, where their hilarious accidental meeting turns into a wild night of partying that turns into — whoa! — the next-morning discovery that during their drunken revelry they’ve gotten hitched.

It’s a positive start to What Happens in Vegas, but it’s only the start of their woes.

Things are civil between them at first on the morning after. But a mutually agreed upon annulment swiftly turns to poison-pill acrimony as tempers flare. Things continue to descend until he drops her quarter into the slot machine she has been playing and wins a $3 million jackpot.

He thinks it’s his.

She thinks it’s hers.

Or is it now, thanks to those wedding rings, community property?

What Happens in Vegas is a battle-of-the-sexes comedy that sometimes hits when it tries to ape the classic screwball comedies of the 1930s, sometimes falls flat when it gets bogged down in the blatant let-it-all-hang-out comedy of today.

Still, it has a plus in its sunny charismatic stars, who, fortunately, develop a solid chemistry. Joy may be controlling and precise, but Diaz lets us see her sensitivity underneath. Jack may seem hopelessly immature, but Kutcher finds his good heart and makes him a sympathetic character who has lived under the lifelong pain of being belittled by his father. Jack and Joy may fight and bicker and pull mean-spirited tricks on each other, but one must be able to see through all that to be able to play along with the conventions of screwball comedy and realize that Jack and Joy are meant for each other. If you can’t get that far, then the film’s inevitable ending will not work. It does work, in fact … but unfortunately director Tom Vaughan can’t resist inserting a couple of ugly moments during the closing credits that nearly sink the good feelings he has built up.

What Happens in Vegas barely skates by on a very thin premise in Dana Fox’s script. In a courtroom, Judge Whopper (no kidding!), played by Dennis Miller, in case you were wondering whatever happened to him, sentences Jack and Joy to six months of “hard marriage.” He wants them to take a shot at making their fly-by-night sham marriage work. He sends them to a marriage counselor (Queen Latifah, at her most beatific). If things don’t work out, they can split the $3 million … which seems to be what they could have done in the first place and avoided all the legal rigmarole. But then there wouldn’t have been a movie.

What follows is the unlikely cohabitation of neat-freak Joy and slob Jack. This should have been the film’s hilarious highlight as Jack and Joy pull dirty tricks on each other, in hopes that the other will throw in the towel, leaving the winner with the entire $3 million. But much of their dirty tricks are childishly silly or, worse, downright gross. Neither character comes out looking good.

Yet as chinks develop in the armor Joy and Jack have built up against each other, What Happens in Vegas begins showing signs of rising above the script’s deadly middle part. A couple of unexpected things happen and all is not a smooth sail to the finish line.

The Daily Show’s Rob Corddry has some funny moments as Jack’s pal, nicknamed Hater, who eggs him on to some of the nastier tricks; Lake Bell scores as Joy’s friend, who views the entire operation with disdain.

But it’s the effervescent Diaz and the easygoing Kutcher who sucker us in with a hate-love relationship that has possibilities.

*** 1/2What Happens in Vegas

Starring: Cameron Diaz, Ashton Kutcher, Rob Corddry, Lake Bell, Dennis Miller, Queen Latifah.

Rated: PG-13, contains adult themes.

mjanuson@projo.com