Ashton Kutcher, left, and Cameron Diaz play reluctant newlyweds in the romantic comedy What Happens in Vegas. (K.C. Bailey/20th Century Fox)

Come on, now. You already know What Happens in Vegas. You've undoubtedly seen the ubiquitous television commercials in which Cameron Diaz and Ashton Kutcher bicker and beat each other black and blue but, secretly, seethe with lust. And you already know that they'll end up softening their stances and falling for each other in the end — it's pretty standard stuff by now. One does not go to a romantic comedy for the Shyamalan-style plot twists.

Director Tom Vaughan's film strives desperately to hearken to those classic screwball comedies of yore, while including the kind of gross-out humour (Kutcher peeing on dirty dishes in the kitchen sink, etc.) that has, unfortunately, become de rigueur for modern-day incarnations of the genre. Which is a bummer because the British director's 2006 feature debut, Starter for 10, starring a then-unknown James McAvoy, was such a charming surprise. What happens in Vegas, however, is exactly what you would expect. It's formulaic, slapsticky, silly and loud, until it goes all gooey in the end.

Sentimentality is not a particularly good fit for either star. Still, Diaz and Kutcher have enough charisma individually and enough spark together to make this otherwise forgettable movie from screenwriter Dana Fox (The Wedding Date) vaguely tolerable. And Lake Bell and Rob Corddry as their respective wisecracking best pals steal some scenes of their own.

Diaz and Kutcher co-star as Joy McNally and Jack Fuller, opposites who meet in Las Vegas and get married after a night of drunken debauchery. While screeching at each other in a hungover stupor the next morning, he hits a $3-million jackpot on a slot machine with one of her quarters. Once they return to New York to fight over the money in divorce court, a judge who's militant about marriage (Dennis Miller) forces them to make it work before either of them can get their hands on a cent.

And so they must live together in Jack's disaster of a bachelor pad. You see, Joy has nowhere else to go. She's just been elaborately dumped by her arrogant fiance (Jason Sudeikis) in front of everyone they know at a surprise party she'd arranged for his birthday. (This is the sort of humiliation that only occurs in movies in general and romantic comedies in particular.)

The fact that they are so incompatible in the cold, hard light of day isn't just uncomfortable for these two, it's dangerous. Despite their disparate backgrounds — she's an uptight commodities trader, he's a party boy whose dad (Treat Williams) just fired him from the family furniture business — they both find creative ways to undermine each other in hopes of grabbing the whole $3 million for themselves. The extreme meanness is actually a good thing — a lot of movies would be afraid to go so far. But it's the hackneyed gags about hair balls in the sink and leaving the toilet seat up that make you really want to groan.

What Happens in Vegas opens across Canada on May 9.