Lightning definitely does not strike twice in "Say It Isn't So," a pale, predictable gross-out comedy/semi-sweet romance in which Farrelly Brothers protégé J.B. Rogers tries way too hard to duplicate the weird and wiley humor that made the Farrellys' "There's Something About Mary" a gargantuan hit.

Limping along on telegraphed bad taste jokes and narcissistic naughtiness, the movie seems to have a scene-by-scene checklist of "Mary" gags to clone (grotesque prosthetic boobs, handicap humor, surprise last-reel celebrity cameo). And for plot, it slogs through one of those scripts that can only move forward if the characters deliberately avoid simple solutions to moronic misunderstandings.

Chris Klein ("Election," "American Pie") stars as Gilly, a Midwestern half-wit who goes gaga over Jo (Heather Graham), his small town's klutzy new hairdresser who cuts off part of his ear during a botched trim.

They have a montage of cute dates and frenzied comedy sex, ending in a marriage proposal. But then Gilly -- who grew up an orphan -- finally gets word of who his real parents are: They're Jo's parents. Ewww!

But before the movie can get any mileage out of that incestuous silliness, we find out it's a case of mistaken identity. Jo's white trash mama (Sally Field, hilariously throwing caution to the wind) wants her daughter to marry rich, so she's taken part in a scheme to hide the truth from Jo, thus driving her back to a loaded old beau in Oregon.

Desperate to win her back, Gilly takes off cross-country while mama convinces Jo her "brother" is a major sex offender determined to get some more taboo nookie. She'll do anything to get her daughter to the church on time with studly Jack (Eddie Cibrian) -- who, of course, turns out to be a two-faced scumbag who sleeps around and runs a drug-smuggling ring.

Undeniably, "Say It Isn't So" boasts the occasional ashamed-to-be-laughing comedy gem. Richard Jenkins (Ben Stiller's inattentive shrink in "Mary") plays Graham's stroke-crippled daddy, who can only talk with the use of a Voc-A-Lator 3000 -- a robotic voice box mama bought at WalMart. Director Rogers milks that gag for all it's worth.

Field steals scenes by playing way, way against type. Fed up with waiting on her invalid husband, when he barks for a sandwich in one scene, she wipes it on her armpits before feeding it to him. Ewww, again!

Current 7-Up spokesman Orlando Jones shows how funny the rest of the movie isn't in his role as a legless hitchhiker -- and whacked-out, Vietnam vet wannabe -- who aids Gilly's plans to crash Jo's wedding.

But most of the jokes here are such duds that you can see the actors stretching for the laugh. And the plot of "Say It Isn't So" is so sloppy the movie can't seem to get from Point A to Point B except by paving the way with random, contrived nonsense.

When Gilly starts getting too close, too early, to finding Jo and telling her the truth, hillbilly henchmen hired by Jack suddenly materialize to knock him unconscious for a the length of a pivotal scene.

The fact that Rogers spends so much energy aping "There's Something About Mary" only amplifies how lame this movie is by comparison. The scenes that attempt to duplicate "Mary's" semi-sincere romanticism come off like impish Hallmark cards. And in "Mary," even the supporting players were much more than the kind of stock character sketches that populate this picture.

But even without such side-by-side scrutiny, "Say It Isn't So" is clearly still a paint-by-numbers affair and a pretender to the comedy-of-the-outrageous crown. Even the reel of bloopers and outtakes that runs with the closing credits falls flat.