There's an unwritten rule about romantic comedies that is pretty much a given. No matter how formulaic the script, if the audience falls in love with the main stars as much as the characters are supposed to fall for each other, then the films work.

That's the case with "What Happens in Vegas." There is not much new here in terms of plot or concept, but thank goodness for Cameron Diaz and especially Ashton Kutcher, who let us in on what is simply a fun-filled romp that hits most of the right notes for a good date flick.

Diaz plays Joy, a high-maintenance New York commodities trader. Her uptight financier fiance (Jason Sudeikis) dumps her in a particularly embarrassing way, and Joy's best pal, Tipper (Lake Bell), talks her totally depressed friend into going to Las Vegas.

Jack Fuller (Kutcher), a party animal and slacker, thinks his job is secure because he works for his father's company. Jack's dad (Treat Williams) challenges him to a one-on-one basketball game -- with Jack's job hanging in the balance. When he loses to his father, Jack is gone.

Jack naturally heads to Vegas with HIS best friend, Steve "Hater" Hader (Rob Corddry). No big surprise, Joy and Jack hook up in a bar, leading to a night filled with WAY too much alcohol.

The next morning the two find themselves in bed, but there's a little twist: Joy quickly discovers a tacky wedding ring on her left hand. They immediately agree to get that marriage annulled.

Then things get complicated, when after a testy chat, Jack throws a quarter she gave him into a slot machine, which hits a $3 million jackpot. Naturally, it becomes a battle of who really has the rights to the newfound fortune.

Back in New York, a judge refuses to grant them a fast divorce and split the moolah. Instead, he demands they live together for six months, see a family therapist (Queen Latifah) and then come back to court. Whoever opts out of that arrangement and leaves loses the cash.

The remainder of the film follows the couple and their schemes to force each other's hand -- leading them to give in and forfeit the money.

Diaz is a delight here, but this is Kutcher's film. He's funny as hell, but also jumps off the screen as the most appealing of leading men when the role calls for that. It's a shame these two talented comedic actors didn't have a stronger vehicle to help drive the laughs.

By Bill Zwecker / Universal Press Syndicate