Billed as the final chapter in the so-called "Wolfpack Trilogy," there were precious few reasons to expect that "The Hangover Part III" would be any improvement over 2009's mean-spirited, unfunny "The Hangover" and 2011's more mean-spirited, less funny—and also shamelessly derivative—"The Hangover Part II." The fact that this second sequel diverges so drastically from the formula of friends blacking out and having to piece together the puzzle of the crazy night before, however, proves to have been a smart decision. Less of a rehash and more a fitting conclusion to the story and characters, "The Hangover Part III" actually surprises on occasion by how involving it is—particularly in the third-act home stretch. It's tough-going until then, littered with cruel and base humor that falls flat, but let's at least give credit where credit is due: for the very first time in the franchise, there are singular moments that approach a genuine sweetness.

Emotionally stunted 42-year-old Alan (Zach Galifianakis) is having a tough time; he has no girlfriend or job prospects, he's become notorious tabloid fodder after causing a freeway accident involving a pet giraffe, and his harried father (Jeffrey Tambor) has just bit the dust. His worried mother (Sondra Currie) sets up an intervention participated in by brother-in-law Doug (Justin Bartha) and buddies Phil (Bradley Cooper) and Stu (Ed Helms), but on their way to take Alan to rehab they are attacked by a drug kingpin named Marshall (John Goodman). It seems that all those years ago during Doug's Vegas-set bachelor party, the psychotic, cocaine-crazed Mr. Chow (Ken Jeong) ripped him off and ran with $21-million. Having just escaped from a Bangkok prison, Chow is now on the loose, and Marshall wants payback. With Doug being held hostage, Phil, Stu and Alan must set out to capture Chow and deliver him to Marshall—or risk the life of their friend.

For fans of "The Hangover" and "The Hangover Part II," it's a tough call how they might react to this third and final series entry, which mostly does away with the raunchy debauchery of the previous movies and goes for a tone that is darker and perhaps even a little more serious. The ironic thing, then, is that this one has more funny moments than the other two combined ("Part II" was a particular dead zone of failed humor), and its differences are what make it the most appealing of the trio. Once again written and directed by Todd Phillips (2010's "Due Date") and co-written by Craig Mazin (2008's "Superhero Movie"), "The Hangover Part III" eventually surpasses tempered expectations, but before this time the film aspires little faith. The opening scene, depicting the gruesome beheading of a giraffe for chuckles, is deplorable, leading one to wonder why such an awful occurrence is meant to be funny. When Phil opines, "He killed a giraffe, who gives a fuck?" it's a safe bet he won't be winning any "Most Likeable" contests anytime soon. Continuing on with the filmmakers' particular grudge toward animals, they also offer up the sight of a chicken being smothered with a pillow. It couldn't be less amusing if it tried. The picture's whole first hour moves along at an awkward pace, full of stops and starts and little continuous momentum.

And then, like a beacon of hope on a fog-drenched evening, "The Hangover Part III" segues from Tijuana to Las Vegas and starts to turn around for the better. A visit to stripper Jade (Heather Graham), now happily married to a surgeon (and pregnant), feels shoehorned in, but Alan's interaction with Jade's 4-year-old son Tyler (adorable newcomer Grant Holmquist) is a charmer. From there, the guys head to a pawn shop off the Vegas Strip where the quirky owner, Cassie (Melissa McCarthy), might just be Alan's soul mate. In a small but savory three-scene role, Melissa McCarthy (2013's "Identity Thief") threatens to steal the film alongside an inspired Zach Galifianakis (2012's "The Campaign"), whose Alan is as out-to-lunch as ever, but somehow more tolerable, even sympathetic, here. With the forever glistening neon illumination of Sin City sumptuously photographed by Lawrence Sher (2012's "The Dictator"), apparently learning a thing or two since the dingy first "The Hangover," the climax transforms into an involving action pic, complete with a perilous, death-defying set-piece outside Caesar's Palace, a car chase, and a "Honeymoon in Vegas"-style scene where Chow parachutes off his hotel balcony.

The mid-credits coda reverts back to the earlier films' propensity for juvenilia and nudity as a punchline, but "The Hangover Part III" one-ups its predecessors by acknowledging a heart beneath the surface of all the empty gags and wild animal slaughter that don't work. Bradley Cooper (2012's "Silver Linings Playbook") and Ed Helms (2012's "Jeff, Who Lives at Home") act as if they're along for the ride, fulfilling an obligation while leaving the heavy lifting for Galifianakis and Ken Jeong (2013's "Pain & Gain"), going-for-broke without being a supreme annoyance as the slippery Mr. Chow. Nevertheless, the unlikely friendship between Phil, Stu and Alan actually, sorta-kinda works because it feels less forced and abrasive somehow. So, yes, in spite of its off-putting elements, "The Hangover Part III" is better than it probably deserves to be. When so many trilogies start strong and finish with a whimper, "The Hangover" movies set the bar very low to begin with, and then lower still, just so the only way to go for the finale would be up. It's an unorthodox tactic not to be advised, but one that "The Hangover Part III" is content to see work in its favor.