Blame it on the jet lag.
Or maybe we should just blame it on Sequelitis Hollywooditis.
It's a common complaint during the summer sequel season, when Hollywood banks -- literally and figuratively -- on viewers buying a brand they've successfully sampled before.
In the case of 2009's "The Hangover," appreciative audiences made it the top-grossing R-rated comedy in Hollywood history.
"The Hangover Part II" ups the ante with a more exotic setting -- Bangkok in, Vegas out -- there's not much here we haven't seen before.
And that's bad news in a movie as desperate for bust-a-gut laughs as "Hangover II" turns out to be.
Alas, there are precious few of those. And every time one of the movie's luckless characters spouts some variation on "This can't be happening again!" you'll be tempted to echo his lament.
I know I was -- more often than I expected. Not that I expected much. (When you've seen as many underwhelming sequels as I have, cynicism tends to set in.)
Still, I certainly wasn't expecting such a listless, been-there, seen-that retread of the original "Hangover" -- which at least had an infectious anything-goes energy that not only reflected its Sin City setting but fueled its twisted humor.
This time, we're ahead of the characters more often than not.
After all, we've seen their act before.
You remember the guys -- and their penchant for getting so blitzed the night before somebody's wedding that they can't recall a thing. Including where they are, what they did the night before and what happened to the one member of their party who's nowhere to be found the morning after whatever they can't remember.
Dentist Stu (meek-and-wild Ed Helms) remembers enough of their Vegas debacle to know that, when it comes to his wedding, he and his pals should stay as far away from the Strip as possible.
Good thing his lovely fiancee Lauren (Jamie Chung) happens to be from Thailand -- and wants her parents to witness the wedding. So their nuptials will take place at an idyllic Thai beach resort.
Naturally, Stu's best friends Phil (slippery smoothie Bradley Cooper) and Doug (straight-arrow Justin Bartha) are invited.
Equally naturally, Alan (a not-quite-sharp Zach Galifianakis ), the arrested-development case who triggered much of the comedic trauma in Vegas, is not. Until Doug's wife, Tracy (Sasha Barrese), successfully pleads his case.
Thus the stage is set for a howlin' "Wolf Pack" reunion -- with the addition of Lauren's brother Teddy, a Stanford pre-med student and cello prodigy. (Teddy's played by Mason Lee, who made his movie debut as a baby in his Oscar-winning father Ang Lee's 1993 breakthrough, "The Wedding Banquet.")
With such an accomplished son, Lauren and Teddy's dad (deadpan Nirut Sirichanya ) is hardly thrilled by the prospect of milquetoast Stu joining the family.
After surviving the pre-wedding dinner, Stu, Phil, Doug, Alan and Teddy wander down to the beach to share a wedding-eve brewski -- and three out of the five awaken, much the worse for wear, in a dimly lit Bangkok flophouse. (Of course it's dimly lit; the power's out.)
Doug, the missing groom during the first "Hangover," is safe this time and back at the resort.
This time, Teddy's the one who's missing -- although one of his fingers seems to be in the possession of a mischievous monkey with a serious cigarette addiction.
Alan seems OK -- or as OK as he ever seems -- despite the fact that his head has been shaved. And Stu has a new facial tattoo, a possible homage to the one worn by their old Vegas pal Mike Tyson.
Speaking of old Vegas pals, it seems that giddy gangsta Mr. Chow (Ken Jeong ) was part of their epic night-before Bangkok bash. And he's more than happy to join their search for Teddy.
Assuming it's not too late. If it is, "Bangkok's got him," as world-weary observers wise to the town's shady ways keep telling them.
Lest we forget, "One night in Bangkok makes a hard man humble -- not much between despair and ecstasy." (Or so it goes in that '80s hit -- which, inevitably, turns up before "Hangover II" arrives at its at-long-last conclusion.)
Director Todd Phillips and fellow screenwriters Craig Mazin ("Superhero Movie," "Scary Movie 3") and Scot Armstrong ("Old School," "Starsky & Hutch") put our hapless heroes through a variety of harried misadventures -- only a few of which possess any edgy energy or genuine comic spark.
Much of the time, the guys slog around soggy Bangkok, sweaty and exhausted, going through the motions as they endure what turns out to be a rather tedious trek -- for all of us.
Contact movie critic Carol Cling at firstname.lastname@example.org or 702-383-0272.