Before the Judd Apatow juggernaut rolled into the megaplex, director Todd Phillips was looking to be the buddy comedy's best buddy courtesy of 2003's "Old School."

Although there have been a couple of missteps in the interim -- 2006's "School for Scoundrels," for one -- he makes a welcome return to form with "The Hangover," a piercingly funny, twisted "whatever-happens-in-Vegas" caper that can't be faulted for not taking full advantage of its R rating.

Armed with a crack ensemble, including a surefire breakout turn by Zach Galifianakis, the Warner Bros. release has all the word-of-mouth makings of a sleeper summer smash.

Phil (Bradley Cooper), Stu (Ed Helms) and the decidedly off Alan (Galifianakis) are unlikely the only groomsmen to have thrown a bachelor blowout in Vegas for their soon-to-be-betrothed pal, remembering nothing the next morning of the previous night's debauchery.

But chances are good that few have awoken hungover in a trashed Caesar's Palace luxury suite among wandering chickens, a crying baby in a closet and a very large tiger belonging to Mike Tyson.

Oh yeah, and the man of the hour (Justin Bartha) has gone missing, along with one of Stu's front teeth.

And then there's the matter of the hospital bracelet around Phil's wrist and the fact the claim ticket they hand to the hotel valet produces a Vegas police car.

Pretty much all will be explained over the course of the outrageous hour-and-a-half -- and what isn't will be quite graphically dealt with over the end credits.

The boundary-testing script, officially credited to the writing team of Jon Lucas and Scott Moore ("Ghosts of Girlfriends Past," "Four Christmases") provides the flexible template, but it's the deadpan comic timing of director Phillips and his terrific cast that's ultimately responsible for "The Hangover's" lasting effect.

And though the performances of de-facto leading man Cooper, Helms (Andy Bernard on "The Office") and Bartha are uniformly affable, the movie serves gonzo Galifianakis much in the same way Will Ferrell busted out after his fearless turn in "Old School."

Also along for the noticeably well-shot (by Lawrence Sher) raucous ride is Heather Graham as a sweetly disposed pole dancer whom Helms has apparently married, and Ken Jeong ("Pineapple Express") as the certifiably unhinged, vengeance-seeking Mr. Chow.

Last but certainly not least, there's Tyson, performing a soon-to-be-immortal rendition of Phil Collins' "In the Air Tonight," accompanying himself on air drums.

Who knew?