Cameron Diaz plays Joy McNally, a Wall Street trader who is intensely career oriented. When her fiancée unexpectedly dumps her, Joy and her best pal Tipper (Lake Bell) head to Las Vegas for a wild girls' weekend. It is there that they meet Jack Fuller (Ashton Kutcher) and his buddy Steve "Hater" Hader (Rob Corddry). Jack is also nursing some wounds after being fired from his job as a furniture maker by his father/boss. He has additionally been called on his playboy ways by his latest conquest.

Joy and Jack end up going out for drinks together, which leads to getting seriously drunk, which leads to a quickie marriage that neither of them fully remembers. The next morning, they agree to end it (with some hostility), but then Jack puts Joy's quarter in a slot machine and ends up winning $3 million. Both claim a certain entitlement to the cash. Back in New York, a local judge (Dennis Miller), offended by the couple's lax attitude toward marriage, freezes the money and forces them to try to make the marriage work for six months. This entails weekly visits to a marriage counselor (Queen Latifah).

Joy moves into her new husband's bachelor pad, which is filled with dirt and clutter. They do everything possible to annoy each other. Some of the things are funny (he removes the toilet seat after she complains about him leaving it up one too many times), while others are just ridiculous (Joy discovers that Jack can default on the judge's ruling by being unfaithful in the marriage, so she throws him a "hot girl party" complete with plenty of "sluts"). The animosity continues to build until…until…they realize that they're falling for each other!

Excuse me while I yawn.

If I had to describe What Happens in Vegas in one word, that word would be "shrill." I don't necessarily mind the endlessly rehashed plot so much as the movie's approach to it. The humor intends to be broad, but it ultimately goes way beyond the definition of that word. Broad humor is fine and dandy; the danger comes from going too far, which is what happens here. The situations Joy and Jack encounter are so extreme that they bear no resemblance to actual human behavior, and therefore we don't really care about the dynamic between them.

Consider, for example, the scene where Joy and Jack meet. They and their friends have accidentally been booked into the same hotel room. Instead of calmly recognizing the situation and discussing it, the half-naked Joy and Tipper begin screaming like banshees and physically attacking Jack and Hater. (How's that for an intelligent portrayal of women on screen?) Later, as Jack and Joy race to a therapy appointment, they dash through Central Park and pass someone picnicking with several loaves of bread. The characters each pick up a loaf and start beating each other over the head. We're supposed to like these characters, although they keep acting like insane people. This kind of thing goes on and on, at an increasingly frantic level, until it becomes tiring.

Onscreen animosity mixed with romantic chemistry is tough to do. The old screwball comedies of the 30's and 40's did it brilliantly, although Clark Gable never needed to clock Claudette Colbert over the head with a loaf of Italian to get a laugh. The style works best when the jokes are in the verbal sparring or when the comedy is pitch black (as in The War of the Roses). When it's all silly slapstick - as it is here - it's a lot harder for the audience to have a rooting interest in the central couple.

Another problem is that Cameron Diaz and Ashton Kutcher have the wrong kind of chemistry. I never saw two people who hated each other; I saw two people who were friends and loved working together. The fun they're having comes across on screen, which effectively kills two-thirds of the movie. I know what some of you will say: "But they're really in love, so that chemistry is perfect!" Not really. Yes, their obvious real-life affinity for one another benefits What Happens in Vegas toward the end; however, if we don't believe that Joy and Jack initially loathe each other, it makes everything that follows feel hollow and forced.

To be fair, there are some semi-witty one-liners and gags along the way, and the stars are talented enough to carry a more ambitious version of this story. Diaz, in particular, seems game to bring some real emotion to her character. Even Kutcher, who I haven't typically been impressed by, suggests a wounded core underneath Jack's misogynistic exterior that the script just barely touches on. These two could potentially light the screen on fire if given three-dimensional roles to play in a plot that made some semblance of sense.

Ultimately, What Happens in Vegas resembles a drunken night in Sin City; when it's over, you don't really remember what happened except that it wasn't good.

( out of four)

What Happens in Vegas is rated PG-13 for some sexual and crude content, and language, including a drug reference. The running time is 1 hour and 38 minutes.