Though only a fitfully entertaining romantic comedy and often not even that, What Happens in Vegas is better by a fair margin than anybody could have expected it to be. Which I use as code for "it stars Ashton Kutcher, yet it is not hopelessly bad." Nor, for that matter, is Kutcher himself hopelessly bad. I would not have predicted this. Indeed, I specifically didn't predict it; I was convinced from the trailer that this was going to be a film with a decently flexible concept, betrayed by the two defiantly uncharismatic stars in the lead roles (Ashton's opposite number in this case is Cameron Diaz, who used to be a beloved romantic comedienne before she fell off the face of the earth). As it turns out, just about the worst thing about the stars is that they are intrinsically blank: adding nothing of value to the film and taking nothing of value away.

Instead, almost all of the heavy lifting is done by the requisite zany friends, played by Rob Corddry and Lake Bell (oddly enough, the latter was last seen classing up yet another toothless rom-com that ended up being much better than it had any reason to be, Over Her Dead Body). But I get ahead of myself.

What happens in Vegas is that Joy McNally (Diaz), a schedule-obsessed Type A who has just been dumped by her fiance, Tool McToolson (Jason Sudeikis; the character's actual name is Mason, not Tool, but I don't supposed I actually had you going), arrives with her friend Tipper (Bell) in an attempt to drown her sorrows and justify the ticket Joy purchased for Mason's birthday. Meanwhile, Jack Fuller (Kutcher) and his improbably-named friend Hater (Corddry) fly in to celebrate/forget the fact that Jack has just been fired - by his father - from his job at a furniture workshop. A wheezy meet cute later (they all got sent to the same room because of a computer hiccup at the hotel), plus a magnificent sum of alcohol, Joy and Jack have hooked up and gotten married. They're just about to part unamicably when Jack wins $3 million on Joy's quarter in a slot machine, and one conservative judge later (Dennis Miller, a sort of backhanded casting coup), they're trapped in matrimony for six months, at which point the money is to be released.

All that is in the trailer, of course, I just thought it was worth reiterating because deep down, minus some of the incidentals (the whole bit about the judge could be a lot less contrived than it is), there's nothing in there that couldn't make a perfectly lovely screwball romance. I'm thinking Jean Arthur and Clark Gable, maybe. Directed by Hawks. I mean, there's nothing within the scenario that is wickedly unpleasant, when we boil it down to: accidentally married couple stays together for the money even though they hate each other. That's a classic set-up. Preston Sturges would have gone to town with that set-up. But What Happens in Vegas doesn't have Preston Sturges or Howard Hawks, or Jean Arthur or Clark Gable. It has two middling stars who have never been especially good at comedy and are both past their career "peaks"; and it has Tom Vaughan, a director of one or two episodes of several HBO series, and some TV movies with titles like Final Demand; and instead of a period that encourages inventive and subversive comedy, it comes out during the Golden Age of Film Market Research. Anything that might accidentally make a picture interesting to watch must be sanded to the bluntest edge possible, and we must never - ever - ever - doubt that Jack and Joy will end up together in the final reel.

By that standard of its genre, What Happens in Vegas is a rousing success: we go to see two people fall in love in spite of a host of silly roadblocks, and end up much better people than they were at the start. Some of the individual situations that the two find themselves in are funny in a highly theoretical way, but Diaz and Kutcher lack the natural sense of comic timing required to make the material spark (Diaz had this ability once, but has apparently lost it); the best they can achieve is a pleasantly forgettable success that relies on you the viewer thinking that, since you are not frowning, you must be having a good time.

Given that so much of the film is such a workaday trifle, it's all the more surprising that there's actually some definite quality to the secondary relationship, that of a horndog pursuing an extremely angry woman, which grows up between Hater and Tipper. Corddry and Bell are not the best comic actors in the world either, but compared to the immeasurably vanilla leads they're more than enough to suggest a whole other plane of comic possibility. It helps that the watery slapstick and computer-generated dialogue that typify most of the script is translated in these characters to actual barbs, witticisms and sarcastic asides that despite (because of) their nastiness, come across as the funniest parts of the movie. While Ashton and Cameron are busy hitting each other with hollow plastic woobie sticks, Corddry and Bell are throwing darts. Actual humor is all the more bracing in the midst of so much genial mediocrity.

There is not, make no mistake, any possible reason for someone to bother with the film if they're not predisposed to the contemporary romantic comedy; even in its best moments there's not enough of substance to the film to make it a more worthwhile comedy than...well, it is May, I suppose, and legitimately funny jokes are to be appreciated wherever they crop up. That said, there just isn't that much meat on this film's bones: it is a fine bit of blockbuster counter-programming, insofar as it's 99 minutes of harmless entertainment for people who don't like CGI. That's not much of a reason to see it, but at least it's entertaining on its own terms; soon enough that will doubtlessly seem like the greatest praise that ever a film received.