The answer, according to Todd Phillips' raucous comedy, is pretty much everything. Or at least that's how it seems when three of the four wake up in a trashed hotel suite with: (1) no memory of the night before; (2) the groom missing; and (3) a tiger in the bathroom. Starting with the smallest of clues -- one guy is sporting a hospital wristband -- the reconstruction begins. The traveling-backward proves to be a good gimmick, breathing new life into a standard night-gone-wild plot; we start with the epic disaster, and follow along as our hapless fellows uncover booze-obliterated memories, all while trying to find the groom. (Think of this as Memento with more yuks and none of the brain-twisting.) Ed Helms (The Office) and Zach Galifianakis, as the dim-bulb brother-of-the-bride, collect most of the laughs, with respectable cameos from Rob Riggle (The Daily Show), Mike Epps and Ken Jeong. Hangover is cheerfully low-brow and vulgar, and some of its jokes could be filed under sexist, racist or otherwise offensive. And in keeping with today's comedy zeitgeist, you'll see more unsightly naked man-flesh than pretty girls disrobed. But if you're looking for easy laughs at guys running around in underpants, this is a keeper. Starts Fri., June 5.