I'm assuming you're referring to the Tamil dubbed version of the movie "The Hangover"!
Here's a useful guide for you:
The movie follows the story of three friends, Phil (Bradley Cooper), Stu (Ed Helms), and Alan (Zach Galifianakis), who travel to Las Vegas for a bachelor party. After a wild night of partying, they wake up the next morning with no memory of what happened the previous night. They soon discover that they are missing their friend Doug (Justin Bartha) and have to retrace their steps to find him.