Sequels that improve on their predecessors are sufficiently rare that one has to acknowledge the achievement even when the original was pretty bad and the second movie is no great shakes, either. That’s the case with “Scooby Doo 2: Monsters Unleashed,” the follow-up to the 2002 special effects extravaganza that brought the cartoon mutt to the live-action world. The first “Scooby Doo” was unrelievedly loud and garish and had a broad streak of coarseness that was truly unpleasant; its plot, moreover, was a muddled mess, making for a sort of cinematic migraine. But though this new flick comes from the identical writer and director (James Gunn and Raja Gosnell) and features the same four human leads (and the same CGI dog), it’s much less irritating. “Scooby Doo 2” has a more straightforwardly silly plot–in which Fred, Daphne, Velma, Shaggy and Scooby are challenged by a mysterious villain who’s resurrected a bevy of their old foes–and it’s executed more skillfully. The picture moves briskly, at a pace that youngsters will appreciate, and though a number of flatulence jokes are sprinkled throughout it, they’re kept to a more moderate level than in the earlier flick. The special effects are just as prevalent this time around, but they’re not as irksome as before–most of them have a sort of “Ghostbusters” homeliness to them. (The exception is the masked ringleader of all the mayhem, a figure so poorly designed and realized that it looks as though it stepped out of a cheap 1940s serial.) The action sequences are staged with greater aplomb (more comedy, less violence), too. And for parents there are even a few mildly sophisticated moments to enjoy, as when Shaggy complains that the other members of Monster, Inc. have excluded him and Scooby from the obligatory musical montage.

But though “Scooby Doo 2” is better than the first picture, that doesn’t mean that it’s really a good movie. However well done, it’s still nothing more than a live-action version of a mediocre old cartoon; and while affectionate and nicely tooled, it’s no more than tolerable. The gang doubtlessly has greater appeal for aficionados than the uninitiated, but basically they’re a pretty dull bunch, whose archness quickly wears out its welcome. Still, it must be admitted that Freddie Prinze, Jr., Sarah Michelle Gellar, Linda Cardellini and Matthew Lillard all do a good job of mimicking their animated versions. Lillard is especially successful in aping the scrawny, perpetually annoying Shaggy–though it may be argued that he was born to play the part. Scooby has always struck me as an even a more tedious figure than his human companions, but the CGI rendering is once again fine. (It does look, though, that the long–and pretty awful–dance sequence the mutt’s been given appears to involve a guy in a costume as well as computer-generated stuff.) But as if to compensate for the woodenness of the leads, the picture features some compensation in the supporting cast. Peter Boyle is quite funny as one of the chief suspects, and Seth Green is nicely unstated as a museum curator who provides potential romantic interest for Velma. Unfortunately, an almost unrecognizable Tim Blake Nelson makes little effect as another of the gang’s old rivals, and Alicia Silverstone is embarrassingly bad as a TV reporter with a vendetta against our heroes.

Like its predecessor, the movie is handsomely mounted, and the candy-colored production (designed by Bill Boes) doesn’t seem quite as oppressive to the eye this time (and it’s nicely caught by cinematographer Oliver Wood). Of course, that might just be the result of familiarity. Even David Newman’s score seems more restrained than before. One could do, however, without the musical numbers–including the one that accompanies the final credits.

Youngsters who enjoy the television cartoons and older viewers who look back on them as one of the pleasures of their youth should take to “Scooby Doo 2.” What’s more important, viewers without any special affection for the old shows needn’t bring a pooper scooper this time around, but that doesn’t mean they should go out of the way to walk this dog, either.