"Paul" should be a simple wacky sci-fi comedy filled with pot humor, unrelenting profanity, gay panic, and dry Brit humor. Instead, the film is primarily constructed as a valentine to the fantasy genre, showing more interest dreaming up inside movie references than one-liners. "Paul" is pure geek bait, an oasis of unadulterated affection for all things sci-fi. The movie bleeds green. Thankfully, in the care of screenwriters/stars Nick Frost and Simon Pegg, the picture casts an amusing intergalactic spell, borrowing a Spielbergian concept and filling it with all sorts of enjoyable absurdity and R-rated mischief.

Two English buddies still reeling from their otherworldly adventures at Comic-Con, Graeme (Simon Pegg) and Clive (Nick Frost) have embarked on an RV tour of American alien encounter landmarks, hoping to soak up some extraterrestrial history to feed their geek brains. Instead of tourist traps, the boys find Paul (voiced by Seth Rogen), an actual alien who needs help from the baffled pair to make his way home again after years spent feeding the military and Hollywood his secrets. Accepting the challenge, Graeme and Clive hit the road with the sociable visitor, forming a bond with Paul that's put to the test when Agent Zoil (Jason Bateman) picks up their trail, speeding across the southwest to capture the prized alien for his enigmatic government boss.

With "Paul," Frost and Pegg, along with director Greg Mottola ("Superbad"), employ a formulaic story of a runaway alien and his human pals as an excuse to monkey with a genre that's informed their cinematic tastes. It's bold and broad, but not exactly a farce, with the screenplay taking time to understand the passions of the characters before embarking on wilder bits of humor. "Paul" might have the appearance of one-note slapstick marathon, but Pegg and Frost shake up the film with their personalized sense of humor, lending the film more of a fanboy personality than a consistently vibrating funny bone.

While muted to a certain extent, "Paul" has a majestically crude comic spirit, spotlighting an alien that enjoys marijuana and causal cursing. A little green jokester, Paul isn't handled with any sort of widescreen awe, instead put forth as a wiseacre with magical healing powers, bonding with his human pals over the mysteries of the universe and the joys of full frontal nudity. Voiced with a pitch-perfect irreverent tone by Rogen (if there was ever a wit built for CG creatures, it's this guy), Paul is an intriguingly profane discovery, painfully aware of human reaction to his appearance and tired of using his powers of invisibility. All he seems to crave is weed and conversation, making the character less of a joke machine and more of an amusing peculiarity. A little green bro.

Expectedly, Frost and Pegg fill their nerdy roles with zeal, having a blast playing second fiddle to a smart-aleck visual effect. The performances are charming instead of overly bothered, with an easy chemistry that helps the film find what passes here for sensitivity. Also merry is Kristen Wiig, playing a one-eyed bible-thumper who craves sinful delights once Paul shares his view of the universe with the frightened woman. Wiig gives the silly business a friendly texture, adding a foul mouth and some needed feminine spark to the acidic merriment.

"Paul" is a very funny picture, but it doesn't always chase the joke, which initially feels like a major tonal mistake. However, what the film lacks in bellylaughs it more than makes for in sheer geekery, with Mottola and the writers filling the frame with references galore, most centered on the cinema of Steven Spielberg. A few of the gags and lines are clunky, delivered self-consciously by the cast, but the majority of the picture creates a swirling sense of sci-fi devotion, conveyed with a heartfelt tone that makes "Paul" feel like an elaborate inside joke most audiences should be able to enjoy.

The references only add to the fun, making the feature a game of homages, which range from the obvious to the terrifyingly particular. You don't have to be a student of sci-fi to get all the jokes, but Pegg and Frost script with a geek POV, creating a passion that helps to elevate the film when it tires of dick and fart jokes.

"Paul" works toward a combustible climax with various enemies out to nab the alien before he reaches his iconic rendezvous point, speeding toward an unexpected climax of confrontation, not overt comedic accentuation. Pegg and Frost have faith in their plot, and that dedication keeps the film engaging and unpredictable, even while it lovingly pilfers from some of the most famous sci-fi blockbusters around.