Most comedy sequels are doomed.
Recapturing the spirit of the original often requires absurd plot contrivances that put the main characters back in a similar situation. It chews up big chunks of time and the laughs are usually few and far between. Even if the sequel is able to avoid this particular pitfall, it is usually forced to turn its attention to lesser characters because the main characters lived happily ever after at the end of the previous film. Often that stretches those lesser characters far beyond the point they were ever intended to be. In short, there's a lot that can go wrong.
If you want to see all of that in one big, ugly mess, well then you should go out and rent The Hangover Part III right away. Mercifully, this is supposed to be the last installment in director Todd Phillips' comedy franchise. I'm not sure I could put myself through a fourth Hangover, even out of the stubborn allegiance to the original that got me through the last two.
Now that three of the four members of the Wolf Pack have been married off to unfortunate women after a bawdy adventure, it is Alan's turn. The quirky Alan, who is played by Zach Galifianakis, is undoubtedly the single funniest character in the whole franchise, but Galifianakis' deadpan weirdness isn't meant to take center stage as it does in Part III. When that deadpan weirdness isn't being played off of the straight men (Bradley Cooper's Phil and Ed Helms' Stu) as regularly it just becomes a series of semi-funny non sequiturs. Supporting character stretched into a starring role? Check.
Ah, but how to get the gang back together again? Well, obviously, you stage an intervention for Alan because he's NOT taking his meds, sending the Wolf Pack on a road trip to rehab (I guess?) that is detoured to Tijuana for a reunion with Ken Jeong's Mr. Chow. Absurdly contrived plot? Yup. Hey, while we're at it, let's make Mr. Chow a main character too? OK. Sure thing. Why not throw in John Goodman as the villain too?
That The Hangover Part III goes through all these paces is bad enough, but it has something else working against it -- the compulsory one-upmanship that took the edginess of the original to dark and depressing places in the first sequel. While Part III is not as sad and unfunny as Part II, it doesn't do much of anything to shake the feeling that Alan, Phil and Stu are actually just terrible people, each in their own uniquely awful way.
Sadly, I don't think this franchise would ever be able to turn things around, even if Part IV did become a reality.