2006's "RV" by way of 2008's "Pineapple Express," "We're the Millers" is a proudly rude, unashamedly vulgar road-cum-drug comedy until it turns all gooey in the home stretch. Episodic but very funny much of the time, the film has been directed by Rawson Marshall Thurber (2004's "Dodgeball: A True Underdog Story") with a good-times flair, while the bevy of screenwriters—Bob Fisher & Steve Faber (2005's "Wedding Crashers") and Sean Anders & John Morris (2010's "Hot Tub Time Machine")—turn to a cavalcade of middle fingers and four-letter words when all else fails. The cast throw themselves into their parts and look to be having a wild time, a feeling that rubs off on the viewer until the story's soft, emotional side takes over. The problem isn't that it fails to be "heart-warming," but that said emotions seem disingenuous coming from these particular rough-edged characters.

When professional pot dealer David Clark (Jason Sudeikis) attempts to stop a mugging and gets robbed in the process, he finds himself in immediate hot water with drug lord Brad Gurdlinger (Ed Helms), a man so wealthy he has recently bought a pet orca for his oversized personal aquarium. To repay his debts and make upwards of a hundred grand in the process, David agrees to be a drug mule, traveling down to Mexico and picking up a stash of marijuana that he must deliver back to Denver in four days' time. Afraid that a single guy won't have a chance crossing the border back into the U.S., David comes up with a hopefully can't-miss plan: he will hire his neighbor, down-on-her-luck stripper Rose (Jennifer Aniston), homeless runaway Casey (Emma Roberts), and friendly-if-awkward latchkey teen Kenny (Will Poulter) to pose as his wife and children on the drug run. Decked out in a rambling motor home, their fake vacation quickly becomes a farce of errors, misunderstandings, and mishaps as they all, in their own sort of way, start to become the makeshift family they never could have expected.

"We're the Millers" is a fierce but fun R-rated comedy that thankfully never cuts corners or chooses a safer route for the sole purpose of receiving a less-restrictive rating. It is what it is, take it or leave it, and it's a sure bet most game audiences are going to want to take it. From Kenny's agreement to give oral favors to a gay Mexican cop (Luis Guzman), to late-night confusion that positions David and Rose in the midst of a couple-swapping game with chirpy fellow RV travelers Don (Nick Offerman) and Edie (Kathryn Hahn), to a lesson on kissing that Kenny receives from "sister" Casey and "mom" Rose, to a highly unfortunate spider bite in a place most would like to keep private, the film goes for broke, refusing to shy away from taboo subjects all the more humorous because they are so goofily provocative. It is in the third act, when the high energy has been mostly exhausted and a round of moralizing and heartfelt declarations take over that the picture loses its way. Turning serious is always tricky in comedies as broad as this one, and director Rawson Marshall Thurber doesn't quite sell it.

Jason Sudeikis (2012's "The Campaign"), best known for his long-running gig on "SNL," needs help carrying the lead duties here. Playing a character who is self-involved, smarmy and virtually tactless, when Sudeikis must finally turn a corner and learn the error of his ways, it feels forced. Granted, David isn't the most warm and genial of protagonists, but the role isn't helped by Sudeikis' chilly purveying attitude. On the other side of the spectrum, Jennifer Aniston (2012's "Wanderlust") is on fire as Rose, her natural comedic intuition and years on "Friends" continuing to pay off. Unlike Sudeikis, Aniston has no trouble playing an easily irritable character while still making her sympathetic, and a scene where she performs an elaborate impromptu striptease in an attempt to save herself from Mexican thugs is an inspired and sexy highlight. As their fictional children Casey and Kenny, Emma Roberts (2011's "Scream 4") and Will Poulter (2010's "The Chronicles of Narnia: The Voyage of the Dawn Treader") blend right into the sharp-witted lunacy, the latter impressively performing Lisa "Left Eye" Lopes' rap during an RV sing-along to TLC's "Waterfalls." Finally, Kathryn Hahn (2012's "The Dictator") continues to naturally flaunt her comic genius in memorable supporting turns, this time as the upbeat, clean-cut, secretly frisky Edie Fitzgerald.

A film about people in need of a family—or, at least, someone to appreciate them—"We're the Millers" uses its drug-running plot as a virtual McGuffin. Indeed, they could just as easily be co-workers at a department store, or strangers on a trip to the Magic Kingdom, and things would basically turn out the same. It is definitely a crowd-pleaser, all but guaranteed to be a sleeper hit of the late-summer movie season even if it is jagged around the edges and tries too hard to be too many things at once by the finale. Judged on how much genuine laughter is earned, however, there is no denying that "We're the Millers" succeeds as a high-spirited variation on "National Lampoon's Vacation," lightweight until one considers just how much weed they have stuffed into every nook and cranny of their motor home. "It's enough to kill Willie Nelson!" David exasperatedly exclaims. In this instance, he's not even joking.