If you're Kevin Smith, you've got to be looking at Judd Apatow's movie comedy empire and thinking, "Dude, what the f--k?"

It must seem kind of unfair. After all, Smith paved the way 14 years ago in Clerks for exactly the kind of movie that has turned Apatow into a brand name. Combining nerdstalgia and a case of arrested-with-no-hope-of-bail adolescence, Smith has cranked out comedies in which the art of cinema has remained stubbornly subservient to big explosive funny-shots of laughter and pushing the envelope of acceptable content (see: bestiality on parade in Clerks II) -- exactly the milieu that Apatow has now conquered. Given his aesthetic sensibility, it's almost mind-boggling that it took Smith this long to set a story -- his new Zack and Miri Make a Porno -- in the world of adult movies. The guy practically invented "comedy porn."

The strange, almost innocent thing about Zack and Miri Make a Porno is that it seems to be an idea as old as Clerks, at least if one is to judge from the premise. Our titular protagonists, Zack Brown (Seth Rogen) and Miri Linky (Elizabeth Banks), are lifelong platonic pals sharing a Pittsburgh apartment that they can barely afford on their service-sector incomes. The threat of imminent eviction coincides with them becoming accidental viral-video stars, leading Zack to a brainstorm: Why not solve their financial woes by creating and starring in their own "adult film?" And because it's 2008, why not make it a fully-costumed sex parody of Star Wars?

Zack and Miri's business model is based on the idea that they can sell the record of their exploits to all of their old high-school classmates at $20 a pop, or perhaps boost that total by somehow convincing those who were willing to watch Miri in "granny panties" for free suddenly to consider coughing up cash to watch her have sex while wearing a Princess Leia hairdo. Throw in the need to recoup production costs, and it's no wonder these people are broke. Their quaint view of the world of smut isn't the stuff of which fortunes are made.

It's clear, however, that Smith is just looking for a thin, naughty excuse on which to hang a tale of two friends who figure out they love each other -- sort of a When Harry F--ked Sally on Camera for Money. In so doing, he ventures into the territory where he always has the most difficulty: capturing actual emotion. It's one thing that most of his supporting cast is minimally competent; frequent Smith collaborators Jeff Anderson (Clerks) and Jason "Jay" Mewes, give actual porn stars reason to hope for big-screen stardom. But while Banks makes for a charmingly neurotic Miri, Rogen simply recycles the potty-mouthed slacker character he has trotted through The 40-Year-Old Virgin, Knocked Up and Pineapple Express. As expertly as he can growl out a filthy one-liner, his ability to convince as an object of affection is waning. Dude needs to come up with another gear, and pronto.

Smith, too, coasts on familiar shtick -- and the fact is, a lot of it still works. He may be ever-eager to find the line between crude and flat-out disgusting -- witness (or don't) the unfortunate fate of cameraman Anderson during one sex scene -- but he also spins vulgarity into some great, mostly unprintable punch lines. And every once in a while, he can even come up with a printable one, like Zack's description of the factors the keep other people from venturing into skinema: "Options, and dignity."

Smith's still funny when the muse strikes him, but it's just too awkward watching him try to build romantic comedy out of sentimentalizing old-school pornography. So profound is his throwback mentality that when we watch Miri bask in the afterglow of her first tryst with Zack, the song Smith throws on the soundtrack is Blondie's 1979 hit "Dreaming." Apatow, whatever problems one might have with his movies, has managed to turn his crude sense of humor into something human and of the moment. Smith remains a guy with a camera and the barest sense of how to use it, still snickering at the same things he did when "Dreaming" was actually on the radio.

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ZACK AND MIRI MAKE A PORNO

** (two out of four stars)

Starring: Seth Rogen, Elizabeth Banks, Jeff Anderson.

Written and directed by Kevin Smith.

Rated R.