Scooby and Shaggy save the day in "Scooby-Doo 2: Monsters Unleashed" -- or to be more precise, they save the movie. The scaredy-cat dog and his whimpering stoner sidekick get all the laughs (and all the "eeewww!" gags), with such disparity that it's as if a different screenwriter (with half the wit) wrote the balance of the movie.

Alas, James Gunn (who wrote the first "Scooby" movie and last week's clever but dumbed-down "Dawn of the Dead" remake) penned the whole thing -- even the paid product placements for Burger King and the 15-minutes-of-fame sing-along cameo by "American Idol" winner Ruben Studdard.

Sigh.

Without much to work with in terms of inspiration (let's be honest, the original cartoon was never terribly clever), Gunn conjures up a surprisingly resourceful plot for "Scooby 2" about havoc being wreaked by the seemingly inexplicable re-animation of every monster ever encountered by the Mystery Inc. gang of (formerly) teenage detectives.

But his dialogue is embarrassingly slapdash (although, thankfully, the wooden Freddie Prinze, Jr. has only about eight lines as dumb preppie leader Fred) and his subplots are lame -- an awkward romance for nerdy Velma (Linda Cardellini) and a tabloid reporter (Alicia Silverstone) trying to ruin Mystery Inc.'s reputation. Every funny in-joke (the gang's former foes all hang out at a waterfront bar called the Faux Ghost) is ruined by the inane marketing spin put on the movie (the Faux Ghost is a hip-hop club, so why does it draw mostly old men who once hollered "those meddling kids!"?).

And although I can hardly blame Gunn for this one, the fashions that pass as personality for oh-so-girlie dingbat Daphne (who has been "Buffy"-ized for fight scenes by Sarah Michelle Gellar) are simply awful. I mean, a poncho, Daphne? A purple striped poncho?

Yet somehow the smoothly CGI-rendered Scooby-Doo and Matthew Lillard's dead-on rendition of Shaggy manage to rescue the movie from the garbage heap with the kind of humor that should, by all rights, sink it even deeper -- namely fart jokes (something director Raja Gosnell proved strangely adept at in the first "Scooby-Doo" movie), cheap slapstick, and the occasional self-aware zinger.

Stumbling across a refrigerator full of unknown chemicals in a bad guy's laboratory, their munchies get the better of them and soon Scooby's morphing into Looney Tunes' Tasmanian Devil and Shaggy's body has turned into a curvy swimsuit model. Later when they're feeling blue for screwing up (what else is new?) the duo mopes outside Mystery Inc. headquarters while an anti-monster weapon is being built inside. "Like, they're totally having a montage in there without us," Shaggy laments.

Because of these two, I left the theater wearing enough of a smile to identify with the little kids in the audience, who were grinning more broadly -- even though I was still annoyed at how obvious it is that big studios couldn't care less about quality when it comes to children's movies.

That "Scooby-Doo 2" was any good at all is a total fluke, what with lines like Daphne's fight-scene "Taste the pain, Mr. Glowy Ugly Thing!" and nonsensical set pieces like a jousting showdown between a black knight ghost on a big black horse and Fred, who mounts a brand new, souped up Indian motorcycle that just happened to be lying around an abandoned mine.

What idiot thinks up this stuff? Oh yeah, James Gunn.