Phil (Bradley Cooper), Doug (Justin Bertha) and Stu (Ed Helms), attempt to get Alan (Zack Galifiankis) into a rehab program. On their way there, Marshall (John Goodman), who was ripped off by Leslie Chow (Ken Jeong), ambushes them. Marshall kidnaps Doug for insurance purposes.
Phil, Doug and Alan eventually find Chow, who tells them that the missing gold is hidden in a house. Phil, Doug, and Alan are double crossed by Chow, and they come to realize that it was actually Marshall's house. Marshall then gives them two days to track down Chow.
They find Chow in the exact hotel in the first hangover film. They also end up meeting Jade (Heather Graham), who is now is married.
In the end, they give Chow to Marshall, only to have Chow kill Marshall. Alan decides to stay in Las Vegas and hook up with Cassie (Melissa McCarthy).
Six months later, Alan and Cassie wed.
Thanks Michael R!