If you’ve ventured into a movie theatre anytime over the last six months or so, you’ve probably caught the extended trailer for Twentieth Century Fox’s first computer-animated feature, “Ice Age”–maybe more than once, in fact. It’s the virtually self-contained little piece about the rat-like squirrel (actually a composite creature called a Scrat, as it turns out) who causes a glacier to collapse by trying to bury a much-prized acorn in it. The bit, whose mute protagonist resembles a frenzied Wile E. Coyote, is delightful even after several viewings, and it opens the feature with a bang. Unhappily, the remainder of the flick isn’t its equal. “Ice Age” is amusing enough, but it’s episodic and derivative, though visually quite fetching. The script doesn’t match the technical polish, so the picture winds up–despite a few near-inspired sequences–is modestly pleasing but ultimately rather slight.

The narrative of “Ice Age,” though set in the distant past, bears a strong resemblance to the recent “Monsters, Inc.” A trio of unlikely beasts–a sad, standoffish mammoth named Manfred (voice by Ray Romano), a wisecracking sloth named Sid (John Leguizamo) and a snarling saber-toothed tiger named Diego (Denis Leary) band together to return a lost human infant to its stone-age tribe. Their motives differ, of course, and one of the beasts in particular is playing a double game, but for the most part they simply go through a succession of adventures, most simply comic but others aiming for poignance or excitement.

Some of the episodes are quite fine, rather like individual Looney Tune shorts–a hilarious bit featuring a group of melon-worshiping dodo birds is probably the best example. There are also offhanded moments that are surprisingly witty (a brief sight gag involving evolution is a case in point, and the periodic reappearances of Scrat are always welcome), and some of the dialogue–especially that of the fast-talking Sid–is abrasively funny. But the mixture of farce and sweetness has a heavy air of calculation about it, and too often the mixture feels more than a trifle bland. At points, moreover, the transitions have a clumsy feel–at one point, when the landscape suddenly shifts and a lot of time seems to have passed, it almost seems as though a chunk of footage has been misplaced. (Some of the muddiness might result from the fact that, according to reports, a segment of the picture involving a major character–a girlfriend for Sid called Sylvia and voiced by Kristen Johnston–was jettisoned before final release.)

Among the voice performers, Leguizamo surely comes off best–his machine-gun delivery and strong characterization make Sid easily the dominant figure of the trio, despite his relatively diminutive size. Romano’s humorously gloomy tones are a nice counterpoint, and Leary puts his sinister sound to good use. As usual in such efforts, the humans are a pretty colorless lot; the kid our heroes are protecting is utterly generic, and all the cooing and smiling in the world can’t make the tyke much more than a prop. Visually the makers have done their work nicely enough, even if the result pales a bit beside “Shrek” or the Pixar product.

So the animation is “Ice Age” is definitely cool, but in terms of story and characterization it’s a rather mushy, a prehistoric retread of “Monsters, Inc.” that doesn’t measure up to its predecessor.