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Pairing gosh-darn-gorgeous America's sweethearts Cameron Diaz and Ashton Kutcher for the first time in a partying-and-pratfalls romantic comedy must've seemed like a no-brainer. And handily, no brain is required to watch this archly contrived and amiable crowd-pleaser, in which broken-hearted New York commodities trader Joy (Diaz) and jobless party animal Jack (Kutcher) wake up married after drowning their sorrows as drunken strangers in Vegas. But before you can say Fools Rush In, Jack has won $3 million on a slot machine with Joy's coin, and they are sentenced to 'six months' hard marriage' by grumpy Judge Whopper (Dennis Miller), on pain of losing the cash.

Screenwriter Dana Fox (The Wedding Date) pops her quarter in the cliché machine, setting the ever-game Diaz and the ever-goofy Kutcher trotting good-naturedly through some shopworn screwball 'sparring comedy' routines (he pees in the kitchen sink, she wakes him up with the blender) as the couple try to force one another to quit the marriage and forfeit the money. Like the similarly far-from-credible How To Lose A Guy In 10 Days, What Happens in Vegas majors in Implausible Insufferable Behaviour, as when Jack's friends beat him up so he can pose as a battered husband.

More surprisingly, what really tanks the movie isn't this - or the mend-your-lover's-life sappiness the movie wedges in between creaking prank - but an unexpected lack of crackle between Diaz and Kutcher. These two beauties romp through Central Park fisticuffs and seaside smooches alike, emanating nothing but photogenic playfulness, despite Diaz being hotly costumed throughout in a teeny-and-tight style best described as Wall Street-walker.

Director Tom Vaughan (Starter For 10) has fun with the Bacchanalian bar-surfing Las Vegas scenes and with the eager malice of the couple's respective best friends (The Daily Show's Rob Corddry revs up as meanly as his moniker - 'Hater'), but once the second-act schmaltz kicks in, you'll be wishing that what happened in Vegas, had just stayed in Vegas.

The bright-light city sure doesn't set this by-the-numbers romance on fire, with Diaz and Kutcher sarky and larky rather than sparky. Die-hard Diaz fans will probably ensure a box-office jackpot, but it's still a busted flush.

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