Manny, Sid, and Diego return in this sequel to the 2002 hit, "Ice Age." But the chills and thrills are gone as the Ice Age begins to melt. In fact, it's melting so rapidly that the trio must band together once more and spread the news that their hospitable paradise is in jeopardy of becoming a water wasteland. On the plus side, "The Meltdown" brings back a lot of familiar voices such as Ray Romano, John Leguizamo, and Denis Leary, while introducing a handful of new ones such as Jay Leno, Seann William Scott, and Queen Latifah. And returning to the director's chair is Carlos Saldanha, who aptly breathed life into the original "Ice Age" and was responsible for last year's animated adventure, "Robots." But unlike "Ice Age" and "Robots," "Meltdown" is missing leadership from its creator, Chris Wedge, who is involved only as the voice of Scrat, the Ice Age squirrel. Not as a co-director. And subsequently, like many sequels, it falls prey to weak story development and insufficient character depth. Although retaining its characteristic visual style, "Ice Age: The Meltdown" is a disappointing and diluted affair.

When we last saw Manny, Sid, and Diego, they had successfully navigated through the Ice Age, rescued and returned a baby Eskimo, narrowly avoided avalanches and other Icelandic mishaps, battled through a saber tooth tiger ambush, and survived the big freeze. Now, due to global warming, that arctic freeze is starting to thaw and the lush valley paradise that they've built is about to come under water. The first observer on the scene is Scrat, the saber toothed squirrel that never seems quite able to bury or protect his cherished acorn without spawning the next natural disaster. In this instance, after climbing a steep ice wall with his acorn intact, Scrat spies a few holes in the surface that are shooting out water. Trying to plug them with each of his limbs is of no use as an even bigger gusher of water bursts out, sending Scrat plummeting below. The herd, in recognition of the potential threat, begins a long migration out of the valley in the hopes of finding a hollow ark that will help them float to safety.

But there are many dangers that lie ahead. Among them is a group of large, prehistoric sea creatures with big teeth. These precursors of the crocodile and shark have been frozen since the beginning of the Ice Age and have now found new life in the water hospitable environment, unlike Manny, Sid, and the water fearing Diego. Additionally, the trek brings with it a certain level of sadness for Manny, who is quick to realize that he just may be the last of his mammoth species. At least, until the trio bumps into Ellie, Eddie, and Crash - a posse of possums. Or are they? Ellie is noticeably a mammoth, but was raised by possums and convinced that she is one. And in spite of their quirks, the newfound trio becomes part of Manny, Sid, and Diego's pack, fighting through vultures and sea creatures, and shifty landscapes. All on a pace to join the rest of their herd, withstand a deadly tidal wave, avoid extinction, and usher in a new, golden Age.

What made the original "Ice Age" different from its predecessor was style and innovation. "Ice Age" introduced a new world, a new set of unorthodox characters, and a beautiful new visual lattice to tell its story. Even though its premise was not very enticing, the filmmaker's love for the characters and their stories was evident, conveyed with genuine emotion such as when the trio discovers a wall of cave drawings depicting the demise of a mammoth and his family. But to be a successful sequel, you have to do things a little differently. You can no longer rely on innovation and style to see you through. You have to expand on the characters and allow them to grow, you have to build a watertight story that has purpose and definition, and you have to introduce new characters and challenges to keep things fresh and exciting. Sadly, "Ice Age: The Meltdown" does only one of these things satisfactorily.

Manny, Sid, Diego, and Scrat all return for new adventures in "Ice Age: The Meltdown." But the new adventures are pretty much ado about nothing. After Scrat loses his acorn again, Manny the mammoth sulks about being the last of his kind, Sid finds a way to annoy a younger generation, and Diego tags along as the lone sidekick. Although there are some new characters such as the prognosticating Fast Tony, the prey-happy vulture Lone Gunslinger, and the female mammoth Ellie, the characters are essentially static, with nothing to do other than conflict with each other. For instance, Manny and Ellie argue over personalities and repopulating the species, Sid pokes fun of Manny's extinction worries and Diego's inability to swim, and Crash and Eddie, two of Ellie's possum pals, find ways to confound everyone. But in every instance, the interactions are encapsulated like Looney Toons' shorts. And when it's all over folks, is there anything we learn about the characters that we didn't already know?

By far, the biggest problem with "The Meltdown" is that its story serves no purpose other than the observance of three likable characters plodding across the frozen and rapidly melting tundra. For sure, this may be simple entertainment for young children, but older viewers will undoubtedly get restless. That's because the film is pieced together as a series of compartmentalized adventures instead of a single, evolving thread. There's Manny and his depression over extinction, Sid's encounter with a worshipping tribe, and Diego's fear of water. Throw in some antagonizing possums and you have a story that feels rather choppy, as if it were put together via cut and paste with no relevancy or commonality to keep audiences in their seats until the very end. Written by Peter Gaulke, Gerry Swallow and Jim Hecht, who were also responsible for "Black Knight" and "Say It Isn't So," "The Meltdown" fails to adequately expand on its universe and the characters within, keeping everything frozen in time.

Global warming, evolution, and extinction - these are all important scientific phenomenon that every child growing up in the 21st Century should know and understand. Yet, while these environmental issues are addressed quite literally, many of the smaller details are glossed over in an effort to make the story work. Of most significance, the Ice Age itself. In reality, the last great Ice Age or Pleistocene epoch lasted almost a million years. And that would mean that Manny, Sid, and Diego have aged quite well. Also, the circle of life dilemma. How can all of these animals co-exist in the same herd? Especially Diego, a bona fide meat eater, living with mammals and vegetarians. And speaking of herds, why is it that Manny, Sid, and Diego are on their own? It's certainly not the definition of a herd. But forget about reality. This is an animated feature and the more you think about these things, the more you must consider the primary target audience. Doing so makes the flaws in continuity and reality easier to ignore.

Even so, the one thing you can't ignore is the story. And for that, "Ice Age: The Meltdown" is precisely what it claims to be - a meltdown. Or in the very least, a stagnant migration. Without guidance from creator Chris Wedge, without expanding and growing the characters, and without a compelling story, "The Meltdown" can't seem to shake the sequel jinx. This, despite the addition of colorful new characters and distinctive visuals. There just isn't much there. For, without a strong story, the film finds itself on thin ice, unable to support a 10-ton mammoth, let alone a 9-ton possum.