There are many things you're probably expecting from The Hangover Part III - a retread of the first two, perhaps, or a series of good, hearty laughs. You aren't getting them.
What you are getting is more challenging, and possibly less entertaining: a threequel that is in itself a critique of its own existence. Not only is it about intervention and rehab - it is its own intervention and at least attempted rehab. Where the other two movies played danger for laughs, this one plays laughs for danger (perhaps they should have set it in Soviet Russia, heeeeeeeh heeeeeeeh heeeeeh). Some will argue that this is an undercurrent which is accidental, but I believe otherwise. Director Todd Phillips got his start doing documentaries on things like fraternity house hazing rituals and self-destructive punk performance artist G.G. Allin, so I think he understands full well what hangover-inducing behavior can really do, and it ain't all laughs.
Breaking from tradition by not beginning with a Danzig song (fear not, Glenn is represented later in the movie), Phillips instantly inverts the expectations of those who've been paying attention by substituting Hanson's "Mmmbop," as enjoyed by Alan (Zach Galafianakis), who's drunk-driving with a giraffe in the back seat. Speeding under a low bridge, he decapitates the animal and causes a massive pile-up; the resulting stress of which causes his father (Jeffrey Tambor) to drop dead. I love dark humor, but this isn't quite that; in opposition to, for example, Anthony Jeselnik daring you to laugh at his molestation jokes, this feels like Phillips deliberately disturbing you by appearing to play these things off as casual jokes in a big-budget blockbuster (while acknowledging that they should not be so played). I may be reaching, but bear with me.
With Alan's behavior having finally caused absolute, irreversible disaster, the family decides he has to go to a mental health facility in Arizona, but they know he'll only go along with it if his Wolfpack buddies are part of the intervention. So Phil (Bradley Cooper), Stu (Ed Helms) and Doug (Justin Bartha) set off on a road trip with Alan in tow, only to be intercepted by some bad guys led by John Goodman, who have convoluted ties to the series' favorite recurring insane criminal, Leslie Chow (Ken Jeong). Bottom line - they take Doug hostage and charge the rest of the gang with finding Chow, knowing that he's maintained a pen-pal correspondence with Alan, and broken out of the Thai prison he was last left in. They have three days, and it's a task that will send them first to Tijuana and then back to Las Vegas.
Alan is the primary protagonist this time around, and it's all about his journey to figure out that toying around with danger ultimately starts getting unhealthy, especially when people can't protect you any more - he's addicted to trouble and is now hitting rock-bottom, having partially set in motion the kidnapping by some of his actions in the previous films. And what's slightly brilliant is the way the movie mirrors this, taking situations that ought to be hilarious by the standards of mainstream comedy and just showing how sad they actually are. I'm no vegan, but when the gang ends up fighting some homicidal roosters to the death because the roosters were force-fed a diet of cocaine and other roosters, it's more depressing and desperate than laugh-inducing.