The phrase “unnecessary sequel” is almost always redundant, but “Ice Age: Dawn of the Dinosaurs,” the third entry in Blue Sky Studios’ lucrative cartoon franchise, seems especially beside the point. The second entry in the series, “Ice Age 2: The Meltdown,” never got within spitting distance of art, but it was more inventive and poignant than it needed to be in order to make a ton of loot, thanks mainly to its gonzo slapstick set pieces (capped by the near-death experience of Scrat the Squirrel and his ascension to a squirrel heaven of cottony clouds and giant acorns) and a story with unexpected emotional heft. Our mammalian heroes fled their homeland to escape a flood; their journey was a goofball odyssey about fear of death and the necessity of overcoming it, and the urge to leave behind something more than paw prints and spoor. This third installment plays like one of a made-for-TV movie reuniting the cast of third-rate sitcom.

Every central critter has issues, none too urgent. The saber-toothed tiger Diego (Denis Leary) is tired of the domesticated life and wants to leave his herbivore pals and go hunting; the mammoth Manny (Ray Romano) has gone into full-on daddy mode in anticipation of his wife, Ellie (Queen Latifah) finally having the child he’s long dreamed of; Sid the Sloth (John Leguizamo) wants to be a daddy (or maybe a mommy) and gets his chance when he finds three T. rex eggs in a cave and oversees their hatching. The plot takes the intrepid four to a valley of dinosaurs that exists beneath the surface of the iced-over tundra. So far, so good: every dinosaur-obsessed kid has fantasized of roaming a land that time forgot. Yet co-directors Carlos Saldanha and Mike Thurmeier and their screenwriters seem detached from the potentially wondrous setting they’ve created.

Problem is, for all the spectacular rushing about (the “Ice Age” movies excel at action, Diego’s “Bullitt”-car-chase-velocity pursuit of an antelope being an especially striking example), we’re aware that there’s no real reason for any of these events to be happening, much less dragging on long enough to justify a feature-length running time. Even kiddie cartoons need internal logic and halfway credible motivation; this movie offers near-nada. Yes, Sid is a doltish innocent, but he’s also a vegetarian mammal who theoretically has at least a smidge of survival instinct; why would he continue to coddle the baby meat-eaters after they tried to eat his friends, much less pal around with them down in dinosaur-land?

The story’s insultingly perfunctory nature (so shallow it makes “Jurassic Park” seem deep) leaches any sense of wonder, not that the filmmakers seem inclined to summon any. The most original and potentially promising subplot envisions a “Moby Dick”-style rivalry between a gigantic, alabaster-skinned super-predator and a vengeance-obsessed weasel (Simon Pegg) who lost an eye battling him. The movie hypes the beast’s eventual appearance, but when it finally emerges into daylight, it’s just a really big dinosaur, more noisy than scary. The razor-toothed fish gliding beneath the ice in the second film were seen mostly in blurry glimpses, but they had a lot more oomph. They were creepily poetic: waterborne grim reapers, emblems of the heroes’ primal fears.

As expected, the only really satisfying bits are the wordless comic interludes charting Scrat’s surprisingly complicated relationship with a saucy female squirrel. She flounces and flirts like Bugs Bunny in drag, hoping to mesmerize him into giving up his, um, nut; then they fall in love, sort of, and settle down together even though they both seem vaguely dissatisfied, confused as to how, exactly, they got to this point, and whether their union came at the expense of true squirrel-hood. (”This is not my beautiful house. This is not my beautiful wife.”) Stitch the Scrat scenes end-to-end and you’d have a sublime, self-contained featurette; here’s hoping the DVD offers that as a menu option.

Matt Zoller Seitz is our guest critic for the month of July.

"Public Enemies" and "Ice Age: Dawn of the Dinosaurs" open in wide release today; "The Beaches of Agnès" opens in New York today and Los Angeles on July 3rd.