Two days before his wedding, Doug, his three friends, and sister Sophie drive to Las Vegas for a wild and memorable stag party. In fact, when the three groomsmen and bridesmaid wake up the next morning, they can't remember a thing; nor can they find Doug. With little time to spare, the four hazy pals try to re-trace their steps and find Doug. During this time feelings arise with Phil and Sophie, but they must focus and find Doug so they can get him back to Los Angeles in time to walk down the aisle. following hangover 2 and 3 also*

It's been two years since the gang known as the Wolfpack narrowly escaped disaster in Bangkok. Now, Phil, Sophie, Stu and Doug have happy lives at home -- Phil and Sophie are engaged to be married, Stu is happily married, and Doug has his own family, but Alan doesn't. Still lacking contentment and a sense of purpose, Alan has gone off his meds and surrendered to his natural impulses. It's up to the other members of the Wolfpack to get him through his personal crisis by embarking on a road trip to the place it all began: Las Vegas.

Two years after the disastrous events in Las Vegas, it is now Stu's turn to walk down the aisle. Since his bride is from Thailand, Stu decides to hold the wedding there. Desperately hoping to avoid the mayhem of Doug's bachelor party, Stu chooses what he thinks will be a safe option: an alcohol-free, prewedding brunch. But this being Thailand, with Sophie, Phil, and Alan again in attendance. A "doctor", two lovebirds, and a man child is a recipe for disaster, and anything can happen.