What happens in Vegas stays in Vegas … especially if after a wild night of partying you can’t remember a thing about what happened the night before.

That’s the premise of the raucously hilarious The Hangover, a buddy comedy that brings four friends to Las Vegas for a bachelor party that none of them can remember the next morning. Except … what about that tiger in the bathroom? Or the baby in the closet? Or the stolen police car? Or the very angry naked man? Or the bat-wielding goons? Or the missing tooth? Or the missing mattress? Or the missing bridegroom for that matter?

The Hangover plays like an insane treasure hunt in which party animal Phil (Bradley Cooper), henpecked Stu (Ed Helms) and the possibly crazy Alan (Zach Galifianakis) try to piece together these clues to come up with some idea of what happened the night before and figure out the whereabouts of bridegroom Doug (Justin Bartha), who is MIA on his wedding day.

The script by writing partners Jon Lucas and Scott Moore, whose most recent output includes Matthew McConaughey’s Ghosts of Girlfriends Past and Reese Witherspoon’s hit Four Christmases, is like some ga-ga puzzle in which one oddball clue leads to something even stranger and sometimes something a little scary. Like those bat-swinging thugs. Or some very upset members of the Las Vegas Police Department. And, by the way, what happened the night before in that wedding chapel? Also figuring into this off-the-wall equation are a stripper, a bag filled with $80,000 in casino winnings, a taser gun and boxer Mike Tyson.

The fun of The Hangover is not only the offbeat situations the boys find themselves in, but in trying to figure out along with them how all these clues are somehow tied together in a way that will lead them to Doug. Director Todd Phillips is the perfect choice for something this loony, yet with a heart, having already proven that he knows his way around sticky relationships (School for Scoundrels) and can handle wild action as well (Starsky & Hutch). Phillips has assembled a terrific ensemble cast with each of the four guys expertly playing off one another while underscoring each one’s idiosyncrasies.

Funniest is Galifianakis’ Alan, the bride’s brother, who gets to play with the character’s very strange quirks. We get the hint near the start when Alan slobbers over Doug to welcome him into the family, later getting very nervous when they park outside a school because he’s under court orders not to come within 200 feet of a school. He’s also not supposed to touch alcohol or to gamble, but you can guess how long those rules will be obeyed.

Helms (who plays Andy Bernard on TV’s The Office) pointedly calls his character “Dr. Stu Price,” even though his friends dismiss the doctor part because he’s “just” a dentist. He’s under the thumb of longtime girlfriend Melissa (Rachael Harris), a shrew who keeps Stu on a very short leash, even though we soon learn that she is no saint. Melissa believes Stu and his friends are off to celebrate Doug’s wedding with a wine-tasting in Napa Valley. Ha ha.

Cooper (of TV’s Alias and Nip/Tuck) plays married schoolteacher Phil. The most normal of the three (besides Doug who is missing through most of the picture), we are supposed to identify with him. A good-looking guy, Phil relishes his one day of freedom in Vegas. Too bad he can’t remember any of its key ingredients.

Insane as much of this seems, in the end everything begins pulling together in the intricately laid-out script that Phillips maneuvers his way through with an unending stream of surprises and wacky sight gags, not to mention jokes that are delivered with expert timing. What happens in Vegas shouldn’t stay in Vegas. Not when it’s this funny.

****The Hangover

Starring: Bradley Cooper, Ed Helms, Zach Galifianakis, Justin Bartha, Rachael Harris.

Rated: R, contains violence, profanity, nudity, sexual situations, drugs.

mjanuson@projo.com