Settling down back in 1995 to watch two impossibly stupid men drive across the United States in a van disguised as a giant dog, few movie audiences would have imagined that what they were seeing would later represent a symbol of despoiled comic innocence. But so it’s proved for Dumb and Dumber, the slapstick farce wherein Jim Carrey and Jeff Daniels essayed a pair of likeable dimwits on a slow and arduous voyage of self-discovery. Relentless in its pursuit of the cheapest laughs possible, it was at once a seismically funny yarn from a pair of gifted directors (the then ascendant Farrelly brothers) and a nadir of lowbrow idiocy.Yet, remarkably, a film has now come along which manages to besmirch its memory. That’s the achievement of Dumb and Dumberer, an odious ”prequel” in which the vulgarian tone of the original has been retained but lent the novel twist of (clever this) not being funny. Not once. Not ever.Simply not being funny, however, is the least of the film’s problems. Make no mistake, by the time the unending stream of listless breast jokes and arbitrary gurning is over, your will to live will be just a memory. But it’s the sheer cynicism of the project that renders it a potential landmark. Nobody from the original is involved; instead, the story takes us back to its heroes’ disastrous schooldays, with a couple of unknown adolescent actors in the place of Carrey and Daniels.As addled executive decisions go, this one takes some beating. For a start, there’s the small but crucial point that, while in Dumb and Dumber we’re laughing at two naifs in a comedy born of sweet-natured absurdism, now we’re being asked to giggle at a pair of children explicitly portrayed as educationally sub-normal, thrown together to make money for the producers. Yet as they admired their newly acquired sequel rights, the producers of Dumb and Dumberer would also have been aware that reuniting their stars would have severely dented their profits, even if Daniels and Carrey had been amenable to the idea. Carrey, for example, has long since graduated to salaries in the region of $20-million.Enter the quick fix of going back in time, thereby avoiding the need to use anyone featured in the original and confirming before a single gag has been cracked (or exhumed) that this was going to be a rare waste of 90 minutes. A sequel might have been different: just occasionally, there is a valid argument for continuing a story, following much-loved characters into new and exciting adventures. The prequel, by comparison, is simply what happens when a producer has already spent all his or her money on lawyers: an artistically bankrupt exercise in milking a few quid from whatever residual fondness an audience might have.Whether it’s Psycho IV (1987’s unlamented portrait of Norman Bates’s childhood) or the entirety of the ”new” Star Wars trilogy, the one thing that marks any prequel is pointlessness. Among the few true golden rules of film is the one which says that, no matter what the genre, a movie should detail the most important and life-changing episodes of its protagonists’ lives. The prequel, however, expects us to believe that there were actually some even more earth-shattering ones directly beforehand, and that the makers of the first film just inexplicably forgot to mention them. Only it doesn’t work: we don’t swallow it. And all that you end up with is a farrago like Dumb and Dumberer — a title in search of a movie.Maybe the iron laws of dramatic structure are a little high-flown for a film in which the narrative centrepiece is characters appearing to smear faeces over themselves. Rarely has one scene acted as such an apt metaphor for a whole film. — Â