Nickelodeon's cheesily-animated "SpongeBob SquarePants" is something of a mini-phenomenon, a small-screen cartoon whose wry, at times adult-aimed humor has gained over a wide fan-base of children, collegians, and even grown-ups. Having never experienced the television series firsthand, "The SpongeBob SquarePants Movie" appears to have captured the same silly tone and low-tech level of animation, albeit at a feature-film length of just under 90 minutes. Directed by Sherm Cohen, Stephen Hillenburg, and Mark Osborne, the movie deceptively wins the audience over in the first hour—yes, even those past the age of 8—with a quick-witted, giddily sly jokiness not anticipated. Alas, the fun cannot be sustained, nor can the meritoriously breezy material, and the third act wears out its welcome to a discouraging degree. "The SpongeBob SquarePants Movie" has been stretched too thin, doing a disservice to exceedingly clever writers whose work becomes muddled surrounded by a lot of visible padding.

In the peaceful underwater town of Bikini Bottom, the spongy, square-panted SpongeBob SquarePants (voiced by Tom Kenny) dreams of getting the managerial position at Mr. Krabs' (Clancy Brown) brand-new spin-off restaurant, Krusty Krab 2. Once he is tragically passed over for the position, SpongeBob has barely had time to drown his sorrows in vats of inebriating ice cream when he and his best friend, starfish Patrick (Bill Fagerbakke), take it upon themselves to travel the dangerous ocean floor to a place called Shell City. There lies the beloved crown of King Neptune (Jeffrey Tambor), stolen by the dastardly Plankton (Mr. Lawrence) in a bid to falsely frame Mr. Krabs, steal his recipes, and turn the fast-food customers into mind-controlled zombies. If SpongeBob can find the courage to complete the mission, with the occasional magical assistance of King Neptune's mermaid daughter, Mindy (Scarlett Johansson), he hopes to bring order back to Bikini Bottom (and earn some respect and possibly the manager's position, in the process).

For quite a while, "The SpongeBob SquarePants Movie" is not only tolerable, but also downright charming. In its irreverent comedic torrents and overall embrace of the juvenile, the picture recalls the feel of "Beavis and Butthead," minus the non-stop crudeness and annoying lead characters. SpongeBob and Patrick may have one-track minds, but they are earnest, determined, and lovably good-natured goofballs that deserve to succeed and be happy. And, although there is nothing particularly objectionable for younger viewers, there are enough scenes and themes that will go over their heads and make the adult audience members wonder to themselves if they have just unknowingly ingested a slew of mind-altering substances. SpongeBob SquarePants and Patrick's stint at ice cream parlor Goofy Goobers, with one sundae after the next turning these two pals into cackling, incomprehensible, bloodshot-eyed drunkards is wacky, to be sure, but no match for a later bizarrely funny sequences where a trip into a tough biker bar's restroom instantly transforms into a madcap, impromptu "bubble party."

By the time SpongeBob and Patrick have been kidnapped from the ocean by a human trapper and brought to the live-action surface to be dried-out, killed, and put on sale (a conflict stolen, and put to cagier use, in 2003's "Finding Nemo"), the quirky sensibilities of "The SpongeBob SquarePants Movie" slow down to a labored walk. And by the time ex-"Baywatch"er David Hasselhoff has entered the proceedings, giving the sponge and starfish a ride on his tanned, muscular backside, the film has all but slowed to a veritable stop. Hasselhoff has become the fall guy when it comes to self-deprecating cameos (2002's "The New Guy," 2004's "Dodgeball: A True Underdog Story," 2004's "A Dirty Shame"), and his appearance has long-since gotten about as moldy as the "Matrix" parodies that used to pop up in every other big-screen comedy a few years back. Hasselhoff cameo aside, the final-third of "The SpongeBob SquarePants Movie" becomes tedious, plodding, and typical. It also stops being funny. As diverting and cute as the movie is to start off, by the end the picture has drained itself of all mirth and merriment.

Simply drawn and animated (no computer-generated imagery here, unless one counts the live-action interactions with these two cartoon characters), "The SpongeBob SquarePants Movie" is almost a nostalgic old-school effort. Even its premise, frivolous and simple, reminds of the kind of animated feature that might have been released fifteen years ago. Now, however, the world has been blessed with the genius that is Pixar and the wonderment of more complex visual marvels, and, unfortunately, these daffy sea creatures pale enormously next to the infinitely superior joys of "The Incredibles" and "The Polar Express." There is simply no reason to seek out "The SpongeBob SquarePants Movie" in theaters—a thinly-plotted, 90-minute episode of a TV show—when there are more worthwhile and invigorating choices for the family out there.