Friends Doug (Justin Bartha), Phil (Bradley Cooper), Stu (Ed Helms), and Alan (Zach Galifianakis) plan to travel to Las Vegas to celebrate Doug's last hours of freedom before he gets married. At first, the trip seems normal, but when the group take shots together, all hell breaks loose. The next morning, their hotel room is destroyed, Phil has a hospital band on his wrist, Alan has a baby with him, Stu is missing a tooth, and the groom-to-be, Doug, is nowhere to be found. It is now up to the group to retrace their steps in order to determine the events of the previous night and find out what happened to Doug just hours before the wedding.
