June 13, 2003 | When is a sequel not a sequel? When it's a "prequel," a critic-speak term coined to describe the likes of Butch and Sundance: The Early Days or The Flintstones in Viva Rock Vegas. Very much like those unmemorable retreads, Dumb and Dumberer: When Harry Met Lloyd is the work of filmmakers who seek to exploit the popularity of a name-brand film by inventing back stories for characters originally played by older, better known and, presumably, better paid actors.

In this case, newcomers Derek Richardson and Eric Christian Olsen are employed as younger versions of the dim bulbs originally played by Jim Carrey and Jeff Daniels in Dumb and Dumber, the imbecilic but astonishingly popular 1994 comedy co-directed by Peter and Bobby Farrelly (who went on to make Kingpin, There's Something About Mary and other, much funnier comedies).

The conspicuous absence of Carrey and Daniels tends to make Dumb and Dumberer look and feel like a down-market, penny-pinched sitcom spin-off (or, worse, a direct-to-video rip-off). Still, the new lead players earn points for the often uncanny accuracy of their mimicry as they play teen-age versions of the earlier movie's grown-up chuckleheads. With almost frightening zeal, Olsen reprises Carrey's chipped-tooth smile and Moe Howard coiffure as Lloyd Christmas, while Richardson, like Daniels, keeps his hair frizzy and his eyes glassy as Harry Dunne. Both actors (especially Olsen) flawlessly replicate the body language and physical shtick of their predecessors.

Director Troy Miller, working from a script he co-wrote with Robert Brener, does relatively little to tinker with the game plan used so successfully by the Farrelly brothers in the '94 crowd-pleaser. Indeed, a few of the prequel's comic set pieces - including a gross-out sight gag in a bathroom and a wink-wink, nudge-nudge ironic ending - are smudged carbons of similar scenes in Dumb and Dumber.

Compared to the aggressively obnoxious '94 original, Dumb and Dumberer is slightly more restrained - i.e., less scatological - and not quite so dependent on borderline sadism. Ironically, however, the new movie possibly will catch more flak, and perhaps even ignite protests, due to its perverse insistence on portraying of Harry and Lloyd as not merely dumb, but mentally challenged.

The wispy plot involves a corrupt high school principal (Eugene Levy) who figures to pocket a $100,000 grant by establishing a phony "special needs" class. Harry and Lloyd are selected as especially needy students, and they're soon joined by "normal" classmates eager to scam their way into a stress-free, no-homework comfort zone.

It's true, of course, that low-brow, gross-out comedies of this sort traditionally wring laughs from subjects and situations that aren't usually joked about in polite company. But gags about special needs programs (and special needs students) in a mainstream popcorn flick may be too much even for the most indulgent audiences to accept. Don't be surprised if there are picket lines at the megaplexes this week.