A blend of “E.T.” and “Alf,” “Paul” is a clever idea played much more broadly and obviously than it needed to be—like a Judd Apatow slobfest. So what could have been a keen, witty take on alien-visitation tropes instead becomes a noisy, strained buddy movie with more lows than highs, in which scatological gags and potty humor exceed wit by a considerable margin, with some sticky sentiment periodically slathered over it all.

The set-up is simple. A couple of British sci-fi nerds, pudgy would-be writer Clive Gollings (Nick Frost) and his best buddy and illustrator Graeme Willy (Simon Pegg) go to California for Comic-Con, intending to complete their fanboy-heaven vacation by taking a road trip in an RV that will take them to all the famous sites in the Southwest, including Area 51 and Roswell. Along the way, however, they encounter Paul (voiced by Seth Rogan), a foul-mouthed little gray man of the E.T. variety. Paul, who’s escaped after years of government custody, persuades the boys to take him to a site where the mother-ship will pick him up for transport home. Of course, agents are hot in pursuit, led by Zoll (Jason Bateman), a tight-lipped Man in Black.

This is a premise that might have been fleshed out with some sharp humor. But Pegg and Frost opt to work in the broadest possible strokes. They insert references to Spielberg movies with all the subtlety of a sledgehammer, apparently fearful that viewers will be too obtuse to recognize anything remotely subtle. One of their major fallback positions is a stream of limp gay gags, initiated by two drearily stereotyped rednecks (David Koechner and Jesse Plemons), that continue throughout the screenplay. And they cut frequently to Zoll’s doofus underlings Haggard and O’Reilly (Bill Hader and Joe Lo Truglio), whose awful slapstick routines are frankly as funny as a crutch.

But all that pales beside the romantic subplot. It’s not that Kisten Wigg isn’t charming as Ruth Buggs, the daughter of an RV-camp owner who goes off with the boys. It’s that she and her father are presented as that easiest of targets, wacko Jesus freaks, and that once Paul persuades her that her fundamentalist beliefs are hooey, she turns into an obscenity-spouting good-time gal. (Her father Moses, played by John Carroll Lynch, is, of course, portrayed as a thuggish, gun-toting, Bible-thumping boor, and his pursuit of the quartet interacts joylessly with that of the FBI.)

And even worse is the big finale, a frantic but oddly tepid chase-and-confrontation reel that involves Blythe Danner, as the earthling Paul first encountered on his arrival, Sigourney Weaver as Zoll’s hard-as-nails boss, and lots more Spielberg allusions to show that Pegg and Frost have seen the same movies we have. There’s a heavy dose of sentiment here, too, as if to prove all the spoofing wasn’t in earnest, after all.

To be fair, there are amusing moments in “Paul”—with Pegg and Frost on hand, one could hardly expect otherwise. But the odd thing about the movie is how completely unadventurous it is. It takes no chances whatever, falling instead into all the easiest traps the basic idea invites, even in terms of its presentation of Clive’s favorite writer, an arrogant, self-promoting hack named Adam Shadowchild played by Jeffrey Tambor in what amounts to a cameo; something could have been done with the character, but after he’s introduced, he’s pretty much dropped. And from a technical perspective it’s decidedly ordinary as well—the crew apparently taking their cue from Greg Mottola, who directs with a lack of imagination and verve that’s probably explained, at least in part, by the near-constant need to integrate the CGI with the live action.

The problem with “Paul” is one of failed expectations. You simply want more from Pegg and Frost than it brings to the table. It’s possible to enjoy this lowbrow send-up of “E.T.” on its Apatow level, but you’re constantly thinking what it might have been, and the chasm between that and what’s actually up on the screen is pretty depressing.