The film revolves around the story of Doug Billings (played by Zach Galifianakis), a quirky and eccentric friend who gets married in Las Vegas. His friends, Phil Wenneck (played by Bradley Cooper), Stu Price (played by Ed Helms), and Alan Garner (played by Zach Galifianakis), decide to celebrate his last night of freedom with a wild party. However, the next morning, they wake up with no memory of what happened the night before and a mysterious hangover.