Four years ago, upstart computer-animation house Blue Sky Studios, a 20th Century Fox acquisition, released its freshman feature effort, Ice Age.

Though the plot was familiar, recalling Monsters, Inc. and Shrek, and the animation less than cutting edge, the three-way dynamic of its odd-couple trio of Manfred the mammoth (Ray Romano), Sid the sloth (John Leguizamo) and Diego the sabertooth (Denis Leary) charmed audiences. Slapstick interludes with Scrat the saber-squirrel and his elusive acorn aided and abetted the fun.

From where I sit, the original Ice Age has aged well. Though not as original or sophisticated as Monsters, Inc. or Shrek, it gets almost as much replay in my household as Monsters, Inc., and far more than Shrek, which gets none at all. In fact, part of Ice Age’s appeal is its very lack of sophistication and irony, a refreshing change from the overly knowing tone of much family entertainment.

For their sophomore effort, Blue Sky went from the prehistoric to the future with Robots, a step forward both technically and creatively. At the same time, Robots raised a question about the creative range of the Blue Sky filmmakers: Do they have the ability to create emotionally persuasive characters that go beyond stereotypes — a key component in Pixar’s success? Can they create story arcs that take their characters into new emotional territory?

With Ice Age 2: The Meltdown, it’s clear both how far Blue Sky has and hasn’t come. Technically, Ice Age 2 is light-years ahead of its predecessor. The Ice Age world looks far richer and more appealing. Fur looks furrier, ice looks icier and all the water everywhere looks real enough to splash in. Visually, the world of the first film was serviceable; the world of this one is a pleasure.

There’s also a new creative exuberance and visual flair to Ice Age 2, an energy that suggests the film was fun to make. The sabertooth’s share of the credit presumably goes to first-time director Carlos Saldanha, co-director of the earlier films (Blue Sky co-founder and two-time lead director Chris Wedge takes an executive producer credit this time). Ice Age 2 manages to be laugh-out-loud funny more often than the original, and Saldanha directs with a maturity and verve that suggests Blue Sky would do well to keep him in the driver’s seat.

At the same time, Ice Age 2 lacks what such a sequel most crucially needs: a reason to revisit the central characters, a fresh take on their relationships, new places for them to go emotionally or dramatically. In a word, what’s lacking is story.

The benchmark here is Pixar’s brilliant Toy Story 2, which raised Woody’s crisis from the first film (what if another toy replaces me in Andy’s heart?) to an entirely new level (what happens when Andy outgrows us all?). It also reversed the Woody-Buzz relationship, with Woody discovering a larger truth about who he is and where he came from, and Buzz the voice of reason recalling Woody to the central values of toydom.

Ice Age 2 tries to take Manny the mammoth beyond his bereaved status quo (his mate and offspring were killed by Neanderthal hunters) by raising the specter of mammoth extinction before introducing a female mammoth, Ellie (Queen Latifah). Will Manny and Ellie, um, save the species from extinction?

Since every movie romance needs a complication, Ellie thinks she’s an opossum — a conceit that is every bit as silly as it sounds and goes nowhere comedically.

“You’re perfect for each other,” Sid the sloth tells Manny in one of the movie’s few great lines. “She’s tons of fun … you’re no fun at all. She completes you.”

Unfortunately, Sid is wrong. Ellie’s character is woefully underdeveloped, and she and Manny have no chemistry at all. Romano gets a few laughs with awkward-lug lines, and kids may think it’s a scream when Manny compliments Ellie on her huge butt. But the filmmakers have no clue how to make their romance appealing or how to make Ellie a worthy love interest.

They have even less idea what to do with poor sabertooth Diego, here reduced to grumpy lines and given a lame fear-of-water conflict that does nothing for him as a character. Even Sid has lost most of his comic mojo, probably because no one is threatening to eat him or stomp on him.

The sad truth is, now that this threesome has more or less accepted each other, they’re not funny any more. There’s little creative juice in the main story, which involves melting ice threatening to flood the valley and drown its inhabitants unless they can reach an immense boat-like fragment of a giant fallen tree trunk, a sort of natural Noah’s ark. Nor do a pair of menacing aquatic monsters preserved in ice from an earlier era add much.

All this puts a lot of pressure on the scene-stealing Scrat, whose breakout popularity in the first film led to a stand-alone short, “Gone Nutty.” Improbably, Scrat exceeds all expectations, taking his slapstick comedy to sublimely creative heights far beyond his earlier appearances. If Scrat’s earlier antics were reminiscent of “Road Runner” and “Tom & Jerry” cartoons, his work here bears comparison to Jackie Chan or Buster Keaton. Almost single-handedly he makes Ice Age 2 worth seeing.

Other parts are also more than the whole. A gonzo musical sequence with a flock of hungry vultures is much funnier than the first film’s survivalist dodos, and plays not so much as a Disney-style production number as a goofy parody of one. Sid has a moment to shine in a wacky though narratively irrelevant set piece involving a tribe of sloths whose intense interest in him is both gratifying and unnerving. And a sequence with the characters dangling from an impossibly precarious column of revolving rocks gets points for sheer complexity.

Yet in the end as Diego makes a pointless speech about Sid’s role in the pack — a speech that comes out of nowhere and is completely unjustified by anything we’ve seen — the film’s limitations come into sharp focus. The Blue Sky folks have mastered slapstick comedy. They’ve learned a lot about moviemaking and visual artistry, too. Technically, they have nothing to prove. Now they need to learn about little things like characters, relationships and stories.

Content advisory: Some moderate cartoon menace and combat; offscreen death of a minor character; flatulence humor; mild innuendo.

Steven D. Greydanus

is editor and chief critic of DecentFilms.com.