What Fletch was to plaid-checked water-cooler wits in the ’80s, what National Lampoon’s Van Wilder was to college-bound douches at the dawn of Dubya, that’s what 2003’s Old School is to Gen-X frat rats — a secret-handshake movie. A shaggy, intermittently hilarious wish-fulfillment nightmare about sorta dissatisfied, sorta middle-aged dudesters trying to capture the Ghost of Keggers Past by running their own frat house, director Todd Phillips’ squirmy comedy struck a nerve with almost every 40-ish daddy I know.

It captured the gnawing suspicion among the newly gray that whatever fun we’re having can’t possibly measure up to the fun we’re supposed to be having — or worse, the fun we were supposed to have had way back when.

To allay those fears in all of us, there’s America’s rec room — the beacon of Las Vegas, promising a wallow in Caligulan depravity (plus comped buffets!). That would make it the perfect setting for an Old School 2 — and in Phillips’ The Hangover, it arguably is. A second slice of three-handed men-will-be-toddlers tomfoolery, just as uneven and almost as funny, this messy, raunchy farce about three groomsmen on a lost-weekend bender in Sin City continues the director’s fascination with the alpha male’s default setting — childhood reversion. To put it another way: This is a movie about three yutzes who go to Vegas for a bachelor party, lose the groom, and wake up face-down in a high rollers’ suite with live chickens, a smoldering armchair and a Jacuzzi full of inflatable livestock.

In the unforgiving light of day — cinematographer Lawrence Sher makes sunny Vegas look like a coated tongue — the three amigos seek the answer to the burning question: Dude, where’s my groom? Ringleader Phil (Bradley Cooper), a teacher who siphons gambling money off his grade-schoolers, has a hospital bracelet, but no memory. Henpecked Stu (Ed Helms), a dentist — sorry, doctor! — all but floss-bound to his suspicious fiancée, has an $800 ATM receipt but is missing a tooth. Found it! It’s in the pocket of Alan (Zach Galifianakis), the bride’s brother and an incorrigible skeev.

As their search for groom Doug (Justin Bartha) leads from cut-rate wedding chapel to no-tell motel, the screenplay, by hot high-concept-of-the-moment team Jon Lucas and Scott Moore (Ghosts of Girlfriends Past, Four Christmases), strives mightily to strew banana peels in their path — most amusing, some merely desperate. A subplot involving ruthless Asian gangsters would be a complete bust, if not for Ken Jeong’s outlandish Riberace mincing as an epicene Mr. Big; a detour to Mike Tyson’s mansion fares somewhat better, though it’s not helped by the champ’s unease at kidding around.

What proves consistent, as it did in Old School, is the chemistry among Hangover’s three species of party mammalia. Cooper, well-cast as the detestable Sack in Wedding Crashers, has the smarmy look of an avocational gynecologist; his Phil is the closest thing the movie has to a straight man, wedged in between a Felix Unger neatnik and an Oscar Madison slob. As the former, Helms uses his wall-mounted Whiffenpoof features to manic effect, with a girlish shriek for each new catastrophe. As the latter, the ursine Galifianakis, a master at detonating sicko one-liners with a slow fuse, adopts a gut-forward toddle and an air of guileless hedonism like a debauched tot with a city-sized Nuk. (He’s left in charge of an abandoned baby, who rides on his chest as a human airbag; the astoundingly inappropriate trick he teaches the little guy leaves no doubt why he’s persona non grata at Chuck E. Cheese.) Together, they form a lopsided portrait of flabby, shabby wannabe machismo — an instant rejoinder to the old taunt “man up.”

That makes them ideal subjects for director Phillips, among whose earliest films was the still-startling Frat House — a 1998 documentary for which the director underwent Hell Week hazing on camera, and emerged with a grudging love-hate for his “brothers” that was not unlike Stockholm Syndrome. Starting with his teen-sex opus Road Trip, he’s given himself perverted little cameos in his features, as if to remind mainstream viewers of his roots in the New York Underground Film Festival and the GG Allin documentary Hated. But the feeling that comes through in his megaplex movies, from Road Trip on, is closer to late, mellow John Waters. He can’t bring himself to push the material into truly outré territory, or to characterize his growth-impaired guys as degenerate creeps (as in the recent Observe and Report) rather than lovable scamps.

If that gives The Hangover a certain failure of nerve, it also makes the movie, like Old School and Road Trip, an unusually palatable entry in a rancid genre (e.g., Very Bad Things). Phillips’ use of his Vegas locale is pretty facile, but at least he draws on the iconography of the celluloid Strip, from Rain Man to Casino, to show his characters struggling to meet the culture-set standards of “what happens in Vegas” — not a bad metaphor for being a man. It would resonate more if The Hangover’s female characters weren’t such nonentities: Either they’re two-timing, ball-busting sluts (Rachael Harris as Stu’s girl deserves hazard pay), easygoing, nurturing whores (Heather Graham’s part evaporates onscreen), or glorified extras. But considering what passes for a man’s, man’s, man’s world in Todd Phillips movies, women may feel grateful for their relative absence.

THE HANGOVER | Directed by TODD PHILLIPS | Written by JON LUCAS and SCOTT MOORE | Produced by PHILLIPS and DAN GOLDBERG | Warner Bros. | Citywide

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