Doug (Justin Bartha) is about to marry his girlfriend. Two days before the wedding, Doug and his friends Phil (Bradley Cooper), Sid (Ed Helms) and Alan (Zach Galifianakis) drove from Los Angeles to Las Vegas to hold a bachelor party for Doug and stayed in a luxury suite at Caesars Palace. One night later, the three found that Doug was missing, a tiger appeared in the bathroom, a baby was in the closet, Sid was missing a tooth, a Simmons mattress was stuck on the spire outside the window, Phil was wearing a hospital bracelet, Sid had an $800 receipt in his pocket, and their car had somehow turned into a police car. The three were panicked and could not remember what happened last night, but they had to quickly solve the mystery within 24 hours to find Doug and return to Los Angeles...