-By Rex Roberts

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Other than its generic title— What Happens in Vegas and Bachelor Party having been taken—The Hangover has everything going for it: a mile-high concept guaranteed to draw in its target demographic; a witty, inventive script with spot-on casting; and opportunities galore for obligatory gross-out humor. Screenwriters Jon Lucas and Scott Moore ( Ghosts of Girlfriends Past) mix slapstick comedy, mystery and suspense to create a cinematic cocktail with a kick—one part Sideways, one part Memento, a jigger of Old School and dash of Road Trip, courtesy of director Todd Phillips. The concoction delights and surprises, there’s little vulgar aftertaste, and it makes you giddy…the perfect summer fizz for a youthful audience too much in the sun.

The set-up is classic. Two days before his wedding, about-to-be-bridegroom Doug (Justin Bartha) heads to Las Vegas with his best buds, schoolteacher Phil (Bradley Cooper) and dentist Stu (Ed Helms), for a last-blast boys’ night out, riding in style in the vintage Mercedes of his empathic future father-in-law (Jeffrey Tambor). Tagging along is Alan (Zach Galifianakis), the bearded black-sheep brother of the bride (Sasha Barrese) best described as inappropriate. Alan is the kind of guy who wears a jock strap in lieu of briefs, carries a man-purse he calls a satchel, and has masturbation much on his mind.

The odd quartet check into a swank suite in Caesar’s Palace, change clothes and head out for a night of good clean fun, first toasting Doug with shots of Jaegermeister atop the hotel roof, the neon lights of Vegas glowing in the dusk. It’s a beautiful moment, and the last one any of them will remember. The next morning, Phil, Stu and Alan awaken in their trashed rooms, wondering where the rooster came from, why the furniture is smoldering, and what happened in the Jacuzzi overflowing with bubbles and blow-up toys. But this run-of-the-mill dissipation rapidly becomes a swamp of despond, as increasingly improbable discoveries make clear last night was no casual carousal but world-class debauchery. And the worst part is, Doug has gone missing…they’ve lost the groom.

Like the notorious joke “The Aristocrats”—each version must be more outrageous than last—The Hangover keeps topping itself, creating situations so outlandish we marvel at the screenwriters’ invention. Not that any one gag or complication is particularly fresh, but the scenario makes them seem so: The guys must recreate the evening’s events from random clues—a hospital wristband, for example—in a race against the clock to find Doug and bring him home to get hitched.

During the course of their investigation, as it were, the boys encounter (no surprise here) a sweet stripper (Heather Graham), a Chinese gangster (Ken Jeong), a two-bit drug dealer (Mike Epps) and other familiar Las Vegas types (including a cameo by an unlikely celebrity who shall remain unnamed, on the slim possibility that the stunt casting hasn’t been ruined by pre-release hype). The Hangover’s humor is bawdy but not mean-spirited and, except for the unfortunate end-credits sequence screened for the press, doesn’t depend on shock value to coax groans from the audience. This is to say, you can be over 50 and laugh your way though the film without feeling punked.