Zach Galifianakis, Bradley Cooper and Ed Helms try to piece together what happened during a wild night in Las Vegas in “The Hangover.”

It’s becoming a subgenre all its own: the Las Vegas memory-loss movie.

First “What Happens In Vegas,” with Cameron Diaz and Ashton Kutcher (himself a veteran of the seminal stoner amnesiac comedy, “Dude, Where’s My Car?”), trying to remember how they woke up as newlyweds. And now “The Hangover,” in which three guys celebrating their buddy’s impending marriage awake in a Caesar’s Palace mega-suite and try to determine what happened to the fourth — the missing groom.

And what’s with that smoldering chair? The clucking chicken? The baby in the closet? Oh yeah, and that tiger in the bathroom?

One of said seriously blotto groomsmen, Stu (Ed Helms), asks another, Phil (Bradley Cooper): “Am I missing a tooth?”

He is.

Clearly, a night of debauchery and drunkeness has transpired. And it’s up to Stu, Phil and Alan (indie comedy dude Zach Galifianakis, pretty much stealing the movie) to retrace their steps, put the pieces back together “Memento”-like, find Doug (Justin Bartha), and hie him to L.A. in time for the wedding.

A loopy, raunchy farce from the same warped mind that brought you “Old School” — director Todd Phillips — “The Hangover” takes male-bonding screwballness and injects it with several hypodermics’ worth of sublime non sequitur humor and random acts of aburdism. Not everything works, but when it does, the film is ridiculously funny.

Galifianakis is the odd man out — he’s the bride-to-be’s brother — tagging along with the three best bros, delivering goofball observations about “Rain Man,” masturbation and the Holocaust. With a face that’s at once full of innocence and psycho menace, the comedian’s deadpan drollery is matched by a knack for fearless (and shameless) physical comedy.

Cooper is the group’s straight man, the friend who dutifully — albeit wincingly — calls the fiancee to inform her that they’ve lost Doug. Helms’ Stu is a dentist in a relationship with the impossibly naggy Melissa (Rachael Harris) — he’s such a quaking milquetoast that he has to lie to her about the friends’ celebration. (They’re in Napa, not Vegas, visiting wineries, not strip clubs.) How he winds up with a pole-dancer played by Heather Graham. … Well, “The Hangover” is a detective story, too. Three stumblebum Sherlocks attempting to solve a mystery of their own making.

Phillips, with a script credited to Jon Lucas and Scott Moore (and with loads of loose-limbed interpretations from the cast), captures the amity among a group of grown men set loose without spouses, girlfriends or sobriety to hinder them. (Or help.) It’s arrested development time, squared.

With cameos from Ken Jeong as a mad-hatter Vegas gangster and pugilist Mike Tyson as Mike Tyson, “The Hangover” pushes the boundaries of good taste, good sense and good will toward man. And you’ll feel good about it all.

Be sure to stay for the credits, with its slideshow of photos filling in the missing — and outrageous — pieces of the night before.