Americans are an alien species, in Simon Pegg and Nick Frost's Paul

On the surface, Simon Pegg and Nick Frost's Paul looks like a drugs-and-sex-addled reimagining of E.T.. But Paul, opening today, is actually also about how America is an alien planet.

Spoilers ahead...

In Paul, Pegg and Frost (who co-wrote the script) play two British nerds who visit San Diego Comic Con and then travel across the U.S. in an R.V., visiting UFO hotspots. And then they meet a real-life alien, the foul-mouthed but extremely genial Paul (voiced by Seth Rogen). He's on the run from the government, which wants to dissect him, and he needs to reconnect with his fellow aliens so he can get off this rock.

You'd expect most of the humor in the film to stem from the culture clash between humans and an alien — especially what with the humans being nerds, with unrealistic expectations of alien life, and so on. But in fact, Paul has been on Earth for decades and already knows everything about our pop culture. (In fact, we find out at one point that he helped to inspire a lot of it — and there's a reason why pop culture versions of aliens all look just like Paul.)

So instead, the big culture clash in the film comes between the two Brits and that most monstrous of alien species... the Yanks.

I think this is the first Pegg/Frost movie that's set in America, rather than Britain, and this changes a lot. Even more than the substitution of Greg Mottola for their usual director, Edgar Wright, the move to America shifts the whole dynamic of their relationship and their humor. Almost everybody that Pegg and Frost meets in America is a redneck, a trailer-dwelling Jesus Freak, a scary gun nut or a government stooge. And that's scarier than any extraterrestrial creature could ever be.

Most of Paul plays out as a chase movie, which is sort of like a siege in a way. Everybody's trying to get at this R.V., which has the two British nerds and their alien friend in it. (And then eventually, the R.V. holds Kristen Wiig's trailer-park princess as well.) So in a sense, the two nerds and the alien are trying to escape from a world that doesn't understand them, and it's a bit of a metaphor for what it's like to be a nerd for real.

Paul becomes sort of a genial mentor and grouchy big brother to the two nerds, expanding their horizons and helping them to loosen up while they help him escape from the government agents and everyone else. Rogen is pretty engaging, doing his usual cuddly stoner shtick as the voice of a cartoony alien, and he complicates the already complicated bromance between Pegg and Frost. The bits where Paul is getting the two guys (and Wiig) to loosen up are the funnest parts, especially the scenes where they're just sitting around a campfire.

Watching Paul, I was left with the impression that Pegg and Frost really wanted this to be their first really big blockbuster hit in the United States. So they teamed up with a talented American director, Mottola, enlisted a ton of great American stars, and tried to make their nerdy, off-kilter humor as mainstream as they could. According to Box Office Mojo, Shaun of the Dead made a respectable $13.5 million Stateside, and Hot Fuzz made $23.6 million. Pegg's solo films, Run Fat Boy Run and How to Lose Friends and Alienate People, didn't do as well here. Paul has to do way better than those other movies, just to justify its estimated $40 million production budget.

So what you have, in Paul, is an attempt at a mainstream film by a couple of creators who are better at speaking to a niche audience. And the result is sort of a gentle, cute comedy with fewer of the rough edges of the earlier Pegg/Frost collaborations.

The most memorable part of the movie, for me, is the part that a lot of other reviewers have singled out as a weak spot: Kristen Wiig's Jesus loving trailer-park girl who realizes her whole worldview is wrong. Ruth's eyes are opened (literally – she has an eye problem, which Paul fixes using his alien powers.) And she realizes that her narrow fundamentalist outlook can't accommodate someone like Paul. So she goes to the other extreme, cussing ineptly and trying to have casual sex, to make up for lost time. It's a neat spin on the idea that meeting an alien would change your world-view, and the most inventive part of the movie.

The rest of the cast is uniformly great, too: Jason Bateman is just as much fun to watch as usual, as the FBI agent hunting the alien, Lorenzo Zoil. And Bill Hader and Joe Lo Truglio are also good as clueless field agents, assigned to help Bateman. When Sigourney Weaver shows up as Bateman's evil boss, she's totally committed to nastiness, and proves once again that she's actually a comedy mastermind. Jane Lynch is cute as a waitress as a UFO truck stop. And Jeffrey Tambor's brief role as a self-aggrandizing science fiction author really is as insane and beautiful as you'd expect.

But a lot of the actual jokes fall a bit flat. I lost count of how many times people mistake Pegg and Frost for a gay couple, but it stops being funny after the first couple times. The nerd in-jokes, like references to Star Trek, Star Wars or the works of Steven Spielberg, are cute, but few of them live up to the reenactment of an iconic Star Trek fight scene early on. The redneck stereotypes are lazy. Most of the funniest bits with Paul are in the trailer.

Basically, Paul is a perfectly sturdy comedy that never becomes epically funny. Pegg and Frost are just as lovable and watchable as ever, and the huge star-studded cast is lots of fun. But it's not in the same league as Pegg and Frost's earlier efforts, by any stretch of the imagination. And I can't help wondering if part of the reason is because these creators are better off making more films in their native land, instead of coming to Planet America.