What Happens in Vegas

This week’s New York-based romantic comedy in which two sworn enemies fall in love after being forced to work on something together is “What Happens in Vegas.” The film is mechanical and obvious (“comfortable and familiar,” a more optimistic critic might say), complete with a leading man named Jack. Jack is what you call your leading man when you don’t feel like coming up with a better name for him. When you buy screenplay-writing software, the leading man is named Jack by default, and it remains that way unless you change it.

This Jack (Ashton Kutcher) is a devil-may-care twentysomething who’s just been fired by his own father (Treat Williams) for being lazy and unreliable. The film tells us that Jack is useless more it shows us; I guess casting Ashton Kutcher in the role means half the work of establishing the character has already been done.

Likewise, there is Joy, a Type-A over-planner who’s played by Cameron Diaz in no-nonsense mode. Joy is a stock broker who’s just been dumped by her boyfriend (Jason Sudeikis) for sucking all the fun out of his life. Joy and her friend Tipper (Lake Bell) go to Vegas to recuperate, while Jack and his friend Hater (Rob Corddry) do the same. The two couples are accidentally assigned to the same room — such kooky hilarity! — and sparks fly.

A night of drinking ensues, and the next morning Jack and Joy discover that they somehow got married. They agree to an annulment — but then Jack puts Joy’s quarter in a slot machine, wins $3 million, and now there’s a dispute over whose money it is. A judge (Dennis Miller) back in New York refuses to grant them their annulment on grounds that are shaky from both a legal standpoint and a storytelling standpoint. He orders them to stay married for six months, see a marriage counselor (Queen Latifah) to prove they’re really trying to make it work, and then we’ll see about a divorce. Why? I dunno. Because there wouldn’t be a movie otherwise, I guess. Personally, I think it would take a constitutional amendment to get me to stay married to Cameron Diaz.

None of this setup makes a bit of sense, obviously. We have not evolved much from the old days, when a madcap farce would simply handcuff two characters together, or when a crazy person’s last will and testament would require someone to get married in 24 hours, or sleep in a haunted house.

Anyway, Jack and Joy must share Jack’s apartment. She is appalled by his slovenliness and his odors; he refuses to change a single thing about himself and even goes out of his way to torment her. They both try to get the other spouse to commit adultery, which would violate the court order. There’s plenty of juvenile one-upsmanship. Jack gets hit in the crotch a lot. So it goes.

In the end, Jack learns to be more grown-up. Joy also admits that she hates being a stock broker. Jack seems to know his about her, even though the movie never even hinted at it. (Maybe it meant to but forgot.) And speaking of forgetting to mention things, Jack’s friend’s name is never mentioned. Then, late in the film, he ignores a cell phone call from “Hater,” and we’re like, “Who the hell is ‘Hater’?” We’re not reading the script, folks. You have to TELL us what people’s names are.

As is often the case, the secondary couple in this halfhearted effort is much funnier than the primary pair. Tipper and Hater despise each other from the moment they meet, and I think the fact that there’s no reason for the hostility is why it’s funny. Their random, cruel asides to one another make me laugh … as does Rob Corddry in general, I guess.

Kutcher and Diaz are likable enough to get a few chuckles here and there, but the generic screenplay by Dana Fox (“The Wedding Date”) doesn’t give them much to work with. Director Tom Vaughan (of the under-appreciated British comedy “Starter for 10”) demonstrates some style in the way he constructs the dizzying debauchery of Vegas, and maybe in one or two of the physical comedy pieces.

This is McDonald’s, friends. You know pretty much what you’re going to get before you go in. I didn’t laugh much, and I never felt even an inkling of “aw, cute!” romance — but I didn’t want to claw my eyes out, either. If that counts as a success in the rom-com business, then I guess “What Happens in Vegas” is a hit.

C (1 hr., 39 min.; PG-13, moderate profanity, some vulgarity and sexual innuendo.)