Surely, whatever tourist board copywriter or marketing guru came up with “What happens in Vegas stays in Vegas” deserves a cut of the profits of every movie set in Sin City these days. Because they’re all about idiots who appear to take that slogan as a command to engage in the most disgusting, most amoral behavior conceivable — and the prepurchased absolution for all their dastardly deeds. As if Vegas wasn’t in all actuality a tacky amusement park for grown-ups who like to pretend they’re debauched and need permission of their spouses before they even think about cutting loose.

The Hangover suffers from precisely that kind of constrained naughtiness: it thinks it’s edgy and envelope-pushing, but there’s nothing terribly risque or dangerous about it. Oh, there’s the brand of nudity that all of sudden Hollywood deems shocking (i.e. male nakedness). There’s the irresponsible consumption of alcohol and other mind-altering substances. And there’s even a flirtation with the criminal elements of Las Vegas society.

But as with most movies aimed at mainstream audiences, there’s nothing at all to threaten the staid, boring, conventional status quo. Of course, this is infuriating when it comes to a movie like The Hangover, which hints at possibilities far more dangerous than it ever dares go near. Just as the Julia Roberts fairy tale Pretty Women morphed from something that was, in its script stage, dark and harrowing to something sweet and cheery, this could-have-been a foray into the seamier side of life; instead, it steers clear of ever getting near anything truly treacherous. The Hangover drives with a seat belt on in a sturdy family sedan and pushes the odometer five miles over the speed limit when, if it had any real balls, it’d be doing 90 on a motorcycle, with no helmet.

The script, by Jon Lucas and Scott Moore, is at its most daring in how it is constructed. Four pals head to Vegas for the bachelor party of one of them, and they wake up the next morning in a trashed hotel suite, massively hungover, with no memory of what happened over the previous 12 hours. No memory at all. Oh, and they’ve lost the groom, Doug (Justin Bartha), and don’t even know where to begin looking for him.

That’s clever: The Hangover is a mystery tale, the guys following up on the few clues they have at hand. Phil (Bradley Cooper), the suave, handsome one, is wearing a hospital bracelet. Stu (Ed Helms), the dorky dentist, is missing a tooth. Alan (Zach Galifianakis), the borderline-retarded one, is missing his pants. There’s a tiger in the bathroom and a baby in the closet. How they retrace their doings of the night before is intriguing, in a narrative sense.

But this is a comedy — or it’s meant to be — and as much as I would have loved for the sense of the sinister inherent in this concept to turn into something deeply, blackly funny (I’m thinking of Very Bad Things as a possible precedent for this), Lucas and Moore and director Todd Phillips go for the easy, cheap laughs, things that will shock a juvenile mind-set — a mother breastfeeding, a fat old man — instead of the things that would have unsettled a more mature one. Some are just plain disturbing without being funny: there are multiple intimations, for some reason that’s never clear, that Alan is a pedophile. Why would a doctor examine a patient while three total strangers are in the room? Why is a taser to the testicles “funny”? As if it knows, somewhere deep down, that it’s cheating, the movie has Stu insist, “You can’t just tase people because you think it’s funny,” but the movie does it anyway.

The Hangover‘s opening credits run over some very fresh angles on Vegas, showing us sides of the city we don’t usually see. And I got my hopes up: could this possibly dare to not be yet another example of harmless pretend-risk, a roller coaster with padding and safety bars? But no. As soon as those credits wrap up, it’s straight on to the postcard views of the Strip and the seat belt ride through fake seediness. Yawn.