Pixar, the most indomitable of all studios and the creative leaders when it comes to quality computer-animated features, can be relied upon to offer audiences something new and different with each release. Not every project of theirs is a total home-run, but taking a chance and only partially succeeding is still more than preferable to disposable pop-culture monstrosities like Dreamworks' "Shark Tale" and "Madagascar." Pixar's latest effort, "Up," is destined to keep the company's winning streak alive, even if it is one of their more decidedly minor works. The first and third acts are dramatically potent and comparable in humanity and thematic richness to 2008's "The Curious Case of Benjamin Button," but director Pete Docter (2001's "Monsters, Inc.") and co-director/screenwriter Bob Peterson (2003's "Finding Nemo"), perhaps not comfortable delving into more serious territory, have overloaded the middle hour with a lot of slapstick and gimmicky animal sidekicks. The two tones never comfortably merge as one.

Since they were kids, 78-year-old Carl Fredrickson (voiced by Edward Asner) and beloved wife Ellie have dreamed of traveling to Paradise Falls, a so-called "lost world" in South America where their childhood hero, 1930s explorer Charles Muntz (Christopher Plummer), once set off for, never to be heard from again. Alas, the cost of living and everyday expenses get in the way of ever saving enough money to make the trip a reality. When Ellie finally passes away, Carl is left alone in the house where they grew old together, surrounded outside by an in-development urban sprawl. He'd rather just be left alone, but after an altercation with a construction worker and a subsequent court hearing, he faces the prospect of having to move into a retirement home called Shady Oaks. Now, he figures, is as good a time as any to finally make that trip to Paradise Falls. Hooking up hundreds of balloons to his chimney, Carl lifts off into the sky—only to discover that lonely 8-year-old wilderness scout Russell (Jordan Nagai) has stowed away under his porch.

It is not giving anything away to mention that Carl and Russell do, indeed, make it to the lost world, and that they face an uphill battle when the house ends up on the wrong side of the cliff, facing the falls. Unfortunately, where "Up" goes from this point is not equal to what has come before it. The 15-minute opening is, by far, the picture's strongest section, a simple, poetic, encompassing portrait of a couple's life together, in the good times and bad. Those viewers old enough to fully understand and relate to love, loss and the passage of time will be in for a tearjerker of a prologue so stunning that it sets one's expectations impossibly high. The bulk of the plot deals with a frothier fantasy story involving a pack of dogs with special collars that vocalize their thoughts, and an old explorer who has lost his mind and will do whatever it takes to capture Kevin, the mythical 15-foot bird he originally set out to find over a half-century ago. Dug (Bob Peterson), the outsider of the canines, befriends Carl and Russell on their mission, as does the rainbow-feathered Kevin. The rest of them, however, work for Muntz, and he is willing to do whatever it takes to get his way.

Too much of "Up" is dedicated to animal comic relief, a misused exotic setting that narrows the premise's potential scope, and a bland human villain whose comeuppance is jarringly harsh and almost irresponsible for a family film. Hearing a dog's realistic thoughts through a vocal translator is neat for a time, and it is humorous how the imposing leader's dial makes him sound like a squeaky chipmunk, but otherwise these gags wear out their welcome and the four-legged characters don't have the substance to back them up. Likewise, with the innovative notion of a house being carried around the world by balloons, it is a shame so much of the running-time is set on the ground, often amidst barren, rocky landscapes.

Where the movie gets its heart is not only in the relationship between Carl and Ellie, but in the unlikely friendship that forms between Carl and Russell. Russell, who misses the close bond he once shared with his dad, finds in Carl a sort of older father figure who he can look up to. In return, Carl discovers it is possible to let other people into his life while making the most of the time he has left. Ellie—the catalyst for the balloon expedition—would want it that way. The climax of the film, set on a zeppelin floating high in the air, generates excitement and a few armrest-clenching moments, but even that is throwaway fodder in comparison to the story's character-based emotional content. When Carl looks through Ellie's scrapbook and finds all the photos that paint the history of their lives, along with a note she wrote to him before she died, it is a genuinely powerful moment. The final scenes, meanwhile, are gentle and sweet.

As one has naturally grown to expect from Pixar, "Up" is lovingly animated, bright in color and hue. Audiences of all ages will be able to enjoy the film, though some may wish they were more taken with it than they really are. The trouble is in the radical tonal shifts, from the most sobering of life lessons to the broadest and silliest of comedic input. Whereas the latter elements are commonplace and only somewhat inspired, the former dramatic aspects are lingering and truthful enough to carry the picture over its rough patches. Pixar has done better in the past, but, until their next masterwork arrives, "Up" will do nicely as the winsome fable it is.