Todd Phillips scored a hit in 2003 with the raucous R-rated comedy Old School, then for some reason moved to tamer waters for Starsky & Hutch and School for Scoundrels, neither of which amounted to anything. Duly chastened, he now returns home with The Hangover, a movie that's as gleefully dirty as Old School, and maybe funnier.

It's a story of friendship, camaraderie, and alcohol. As such, it can only be set in Las Vegas. That is where Doug (Justin Bartha), who is getting married Sunday, is taken by his friends for an epic bachelor party: Phil (Bradley Cooper), a schoolteacher who hates his students almost as much as he hates being married, is the ringleader; Stu (Ed Helms), a wimpy dentist with a controlling shrew for a girlfriend, is the nervous nellie; Alan (Zach Galifianakis), Doug's soon-to-be brother-in-law, is the spacey, grubby, possibly mentally handicapped one who recalls Will Ferrell in Old School (which means he also recalls John Belushi in Animal House).

The four embark on a night of revelry. The next morning, Phil, Stu, and Alan wake up in their hotel suite with a tiger and a baby. There's no sign of Doug, the groom-to-be. And no one can remember anything that happened the night before.

As comedy premises go, this one is brilliantly simple. Three hungover guys stumble around Las Vegas in search of clues as to their missing friend's whereabouts, not to mention the origin of the baby and the tiger, and not to mention the other various things that have gone awry that, well, I won't mention. (Spoiler: Ed Helms is missing that tooth in real life.)