Two days before his wedding, Doug (Justin Bartha) drives to Las Vegas with his best buddies Phil and Stu (Bradley Cooper, Ed Helms) and his future brother-in-law Alan (Zach Galifianakis), for a blow-out bachelor party they vow they'll never forget.

But when the three groomsmen wake up the next morning with pounding headaches, they can't remember a thing. Their luxury hotel suite is beyond trashed and the groom is nowhere to be found.

With no clue about what happened and little time to spare, the trio must attempt to retrace their bad decisions from the night before in order to figure out where things went wrong in the hopes of finding Doug and getting him back to L.A. in time for his wedding.

But the more they begin to uncover, the more they realize just how much trouble they're really in.