Jason Sudekis stars as Dave, a pot dealer who lives in Denver. He sells pot for Brad Gurdlinger (Ed Helms), and when his stash and money is stolen by muggers, they only way he can pay Brad back is to smuggle a bunch of dope across the border from Mexico. His plan is to recruit a stripper (Jennifer Aniston), a homeless teen (Emma Roberts) and a nerdy virgin (Will Poulter) to pretend to be his family. He figures that will make him less suspicious to the border guards.
This starts out as a very funny dark comedy. They're only doing this for money, and they don't really like each other much. But along the way, they discover that they really like each other and they want to be family. By the end, the movie expects us to care for these unlikeable characters. Which isn't so bad, but they stop behaving the way they really would in this situation.
For example, there is a point where Dave has to get the drugs back to Denver by 9pm that night. They're currently stuck in Phoenix and the kid has been bitten by a spider. They take the kid to the hospital, and all Dave has to do is leave the kid there, drive the drugs back to Denver, then come back the next day and pick the kid up. But he doesn't bring this up at all. He leaves the 'family', has a change of heart, and comes back. It's not like he's leaving the kid stranded in the middle of nowhere, and he stands to lose something like $500,000 if he doesn't deliver. I don't mind the characters starting to like each other, but their behavior doesn't make sense.
Along the way, the 'family' keeps running into a real family in another RV (Nick Offerman, Kathryn Hahn, and Molly Quinn). These guys just about steal the movie, Offerman in particular.
The movie tries to hard to provide a happy ending with everything working out just right. But for most of the running time, there are some pretty good gags. The funniest part is when the kid is being shown how to kiss a girl by Roberts and Aniston. You can easily predict what happens, but it's still a pretty good laugh.