Derek Richardson and Eric Christian Olsen could win the top prize at a school talent show for their worshipful impersonations of Harry Dunne and Lloyd Christmas, the sublime idiots memorably played by Jeff Daniels and Jim Carrey in Peter and Bobby Farrelly's seminally silly comedy ''Dumb and Dumber.'' But giving the pair of unknowns their own karaoke machine of a movie in which to work up their routines is an award to be declined. Dumb and Dumberer: When Harry Met Lloyd -- an opportunistic ''prequel'' to the 1994 box office hit that's as strained as its title -- puts Richardson (as Harry) and Olsen (as Lloyd) in the unenviable position of reminding us that they are, indeed, not Daniels (as Harry) and Carrey (as Lloyd). And that if we wanted Daniels and Carrey, we would have rented the original. And that none of the Farrelly brothers' imitators has ever satisfactorily replicated their notorious comedy recipe for genially crude anarchy.
Those ''D&D'' enthusiasts who have always wanted to learn how young Lloyd cracked a front tooth will have that burning question answered. Faced with no other scorching queries, though, screenwriters Troy Miller (who also directed) and Robert Brenner nevertheless elect to inform uncurious viewers that Lloyd's adoptive father (Luis Guzmán) is a high school custodian, that Harry was homeschooled until the age of 17 by his mother (Mimi Rogers), and that when Harry met Lloyd, the latter confidently warned the former that ''chicks are for fags.''
What plot there is involves the machinations of a weaselly school principal (the hope-inspiring Eugene Levy, but even he can't find the ignition switch to turn on the fun), who, with Cheri Oteri as a lunch-lady sidekick, schemes to embezzle grant money by setting up a phony ''special needs'' class, for which Harry and Lloyd are model candidates. Richardson competently suggests Daniels' languid density as Harry. Olsen appropriates Carrey's rubbery mania as Lloyd. And when all else fails, the filmmakers insert a couple of gross-out scenes involving Bob Saget and substances that look a whole lot like feces.
There's plenty of infantile poo humor in ''Dumb and Dumberer,'' but nothing anywhere near sex, adult or adolescent, and the lack of it pretty well blows the sticky beauty of Farrelly sex farces: The genuine article traditionally matches goobery guys with sparkling, game girls, and dangles the irresistible hope that a cornflower-lovely heroine -- played in the past by Cameron Diaz, Gwyneth Paltrow, Renée Zellweger, and, of course, Lauren Holly as the object of the original Lloyd's ardor -- could really fall for the charms of a doofus hero.
In a final bit of pale imitation, a placid Guess? Jeans model and first-time actress, Rachel Nichols, fills the role of generic Pretty Girl, playing a student reporter investigating the grant scam. But without any of the patented Farrelly insight into the insecure, horndoggy teen in every man, and without a grown-up setting in which Harry and Lloyd can transgress like dum-dum geniuses, ''Dumb and Dumberer'' is dumberest.