Two days before his wedding with the wealthy Tracy Garner(Sasha Barrese), Doug Billings(Justin Bartha) travels to his bachelor party in Las Vegas with his best friends, the school teacher Phil Wenneck(Bradley Cooper) and the dentist Stu Price(Ed Helms), and Tracy's unconventional brother Alan Garner(Zach Galifianakis). Tracy's father lends his convertible Mercedez Benz that is his pride and joy to Doug with many recommendations. The quartet rents the best suite in the Caesar's Palace and they go to the roof to celebrate. Alan proposes a toast to Doug and they drink his booze. On the next morning, Phil, Stu and Alan have a hangover and they do not recall what they did last night; the room is upside down; Doug is missing; there is a baby in the wardrobe; a tiger in the bedroom; a chicken in the room; Stu has a missing tooth; and Doug is missing. The trio tries to track down last night to find what happened and sooner they discover that Stu married the stripper Jade (Heather Graham) and Alan has spiked their booze with "Good Morning, Cinderella" in the beginning of their crazy night.