The good news is that “The Hangover Part III” isn’t a rerun like the second episode. The bad news is everything else. For all the promise of mayhem and WTF moments, the final episode hits you with all the force of a warm can of O’Doul’s.

When basing your movie on four characters, you really shouldn’t have two of them (Bradley Cooper’s Phil, Ed Helms’ Stu) stand around being boring and reacting to things. As for the uselessly strange Zach Galifianakis, who hasn’t made me laugh since 2007, his Alan is stupid in boring ways, while even the talented Ken Jeong can do nothing with Chow, the laboriously wacky Chinese guy who serves as the fuse on this soggy firecracker of a movie.

There’s no spree/hangover this time, just a failed intervention during which the fellas try to take Alan to a rehab clinic (they have that for a case of dumbass?). Instead all are kidnapped by a gangster (John Goodman) linked to the dealer who sold Alan drugs in the first film.

Goodman’s Marshall tells the guys that Alan’s friend Chow, who has recently escaped from a Thai prison, robbed him of $21 million in gold bricks (which, according to this movie, can be easily lugged around in three or four duffle bags but even more easily stolen from a sheik who brings it on a plane with him with less security than your average 7-Eleven). Marshall says if the Wolfpack doesn’t bring him Chow, he’ll kill their pal Doug, who again is played by the woebegone nonentity Justin Bartha. In the three films combined he gets about as much screen time as Rebecca in “Rebecca.”

Chow is in Tijuana, so the friends head to Mexico, where Chow helps them break into a house he says he used to own where the gold is hidden (Though since it’s not actually his house, it’s a mystery why he knows everything about it.)

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In the second half of the movie, we bounce back to Vegas to rappel off the roof of Caesars Palace and chase a parachuting Chow. Randomly thrown in are “trailer moments” unconnected to anything: Why is Alan driving down a freeway with a giraffe on a trailer and heading for a low underpass? No reason, because that would require actual writing instead of comedy hacks sitting around asking each other, “Wouldn’t this be off the hook?” Why does Alan sing “Ave Maria” in a gorgeous soprano? See previous. Why do the kidnappers wear Porky Pig masks if they’re going to reveal themselves anyway? Ditto.

I beg of you, don’t ask me to choose between “Hangover” II and III. That would be like asking a mother to choose between her children, assuming she hated her children, never wanted to see them again and wished they’d never been born in the first place. Part II was like being No. 68 for takeoff at LaGuardia on a parked plane with the air conditioning off while an overcaffeinated psychotic in the seat next to you keeps repeating himself.

Part III is more like being a hundred feet from the punch bowl at your class reunion and trapped in the corner by the most insistently dull guy in your high school class as he regales you in pathetically vast detail about the habits of his extensive collection of pet ferrets.

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