Doug (Justin Bartha) is about to be married, so his friends -- Stu (Ed Helms), a dentist planning on proposing to his girlfriend of three years, Phil (Bradley Cooper), a schoolteacher bored of the married life, and Doug's soon to be brother-in-law Alan (Zach Galifianakis), who is smart yet socially-inept -- take him to Las Vegas for a bachelor party. Stu's girlfriend is very controlling, thereby forcing Stu to lie to her about their destination.
The guys get a suite at their hotel, then sneak on to the roof of the hotel and toast to the night ahead. Skipping forward to the next morning, the three groomsmen awake with no memory of the previous night. They find Doug missing, Stu missing a tooth, a tiger in the bathroom, and a baby in the closet. Initially they believe Doug to have gone out for breakfast, but soon worry when his cellphone has been left behind. They collect what memories they have to figure what happened to Doug. The valet brings them a police car, which they had dropped off the night before.
Because Phil is wearing a hospital bracelet, the guys head to the hospital. The doctor identifies roofies in their blood, and tells them that they had just come from a wedding. At the chapel, they learn that Stu had married a girl named Jade (Heather Graham). On the way to her place they're attacked by two Chinese thugs. They escape and find her, where she reveals she's a hooker and the baby is her's. Cops bust into her apartment to arrest the three men for stealing their car. However, not wanting to face the embarassment of losing their car, they work out a deal in which the men volunteer to demonstrate how tasers are used. Afterwards they pick up their car in good shape, though a Chinese man named Mr. Chow (Ken Jeong), is locked in the trunk and attacks them upon his release. Alan admits to having drugged their drinks the night before, hoping they'd have a better time, but had been sold Roofies by accident.
They head back to the hotel to look for further evidence, but discover Mike Tyson instead, who is the owner of the tiger in their bathroom. He forces them to bring the tiger back to his place, and on the way there it destroys their car. After returning the tiger they're attacked again by Mr. Chow and his men. According to Chow, they'd mixed up bags the night before, and they were holding $80,000 of Chow's money. Chow demands it back in exchange for Doug, whom he has apparently captured. Unable to find the money, Alan decides to use his knowledge of counting cards to win the money in Blackjack. The money is returned, though Chow had captured a different Doug (Mike Epps), who was the dealer that had sold the Roofies to Alan.
Phil is in the middle of calling Doug's fiance to tell her they lost Doug, when Stu realizes where Doug is. The guys had seen a mattress flung from their hotel earlier, and Stu deduces that Doug is locked on the roof. They find him, sunburned but holding on to the $80,000. They rush back home and make it to the wedding just in time. Phil happily goes back to his family, whilst Stu breaks up with his girlfriend and arranges a date with Jade. Alan finds a camera they'd had with them, & together they look at the images from the night they'd forgotten.[D-Man2010]
