While it's true that The Hangover, Part III doesn't exactly duplicate the plot of the original Hangover, the way the atrocious Part II did, it doesn't bring anything much new to the screen, either. Doug (Justin Bartha) is kidnapped by a mob boss named Marshall (John Goodman), in order to convince the Wolfpack (Bradley Cooper, Ed Helms, and Zach Galifianakis) to chase down Mr. Chow (Ken Jeong), who has stolen millions of dollars' worth of gold bars from Marshall. It gets more implausible from there, with a series of heists and stunts and a chase scene or two, all around Tijuana and Las Vegas. I laughed out loud a couple of times—is it telling that the best jokes in the movie have to do with cocaine, I wonder?—but mostly, I just wanted it to be over.
The problem is that it's the same humor as the other two Hangover movies, over and over again. Ken Jeong acts batshit crazy, Zach Galifianakis is inappropriate to just about everyone, and Bradley Cooper and Ed Helms get all the "can you believe this shit" lines, playing the fussy adults who have to chauffeur the hyperactive kids from one set piece to another. But they're not even given character arcs this time, unless you count bugging your eyes out and shouting "ALAN!" over and over again as a symbol of emotional growth.
None of this would matter if there were a lot of crazy humor in The Hangover, Part III, but there just isn't. The film suffers from great comedic droughts throughout, especially when director/cowriter Todd Phillips tries extra-hard to connect this final chapter to the other two films in the trilogy like it's all part of some grand plan. On the contrary, it should be obvious to everyone now that the first Hangover movie was a freak accident, a goofy comedy that juiced itself up on the unexpectedly vivacious chemistry of the cast. The second one was a desperate, greedy attempt to re-create that chemistry. And this third movie feels like everyone involved realized it was a terrible idea halfway through the filming of it, but pride—not to mention a studio insistent on filling a hole in its summer release schedule—demanded that they drag themselves across the finish line. They do, just barely.