IT'S RUGRAT-ER-IFFIC!

Chuckie Finster: a messy-haired, bespectacled two-year-old who is pessimistic, fearful and yet strangely endearing--a baby Woody Allen.

Phil and Lil De Ville: a set of no-non-sense twins who can only be told apart by the bow Lil wears in her single shock of brown hair, and by the fact that Phil wears pants.

Tommy Pickles: our hero, and "the bravest baby I ever knowed," according to his best friend Chuckie. Bravery, courage, guts--whatever it's called, one feels that Tommy's diapers hide more than just the poop which is so often mentioned in the movie.

This is the basic cast of "Rugrats," one of Nickelodeon's most popular cartoons. In every episode, Tommy, Chuckie, Phil and Lil take on the rest of the world in fun adventures and tackle surprisingly adult issues in a manner which still leaves room for poop jokes. The great thing about the series is that while small children love the simplicity of the adventures, the show also throws in enough sophisticated humor for older viewers to have a good time watching it.

Which is part of the problem with the movie: the adventure aspect of the movie grows to be so sprawling that there is no room left for the delicate humor that older viewers will appreciate. The Rugrats Movie starts out harmlessly enough. "Rugrats" fans of all ages will be thrilled by the entrance of a new cast member: Dylan Pickles, who is promptly and unfortunately nicknamed "Dil" by his very own parents. He turns out to be just as sour as his namesake, much to older brother Tommy's dismay. To allay Tommy's miseries, his father Stu takes him to the basement and bestows "responsibility" upon him in the form of an heirloom pocket watch; as a big brother, Stu tells Tommy, he now has a responsibility to take care of Dil. Tommy is awed by the watch his father has given him, and takes good care of the "sponsitility," as he calls it, throughout the movie. Never mind the fact that Tommy is all of two years old and more likely to drool on it than appreciate its antique beauty--anything can happen in a Rugrat's world.

And most of it does happen. When Dil's constant screams and crying grow to be too much for the original Rugrats, Phil and Lil, always brutally practical, suggest they take Dil back to "the baby store" for a refund. After all, they reason, why keep a "broked" baby when they can get a new one that works better? Despite his misgivings, Tommy must go along for the ride to care for his younger brother as Chuckie and the twins take Dil back to the hospital in the Reptar Wagon, a crazy contraption built by Stu in his toy workshop.

After an exceedingly well-executed song-and-dance sequence involving a large cast of newborn babies and a whole lot of urine, the movie falls into something of a rut. The adventure gets a little too ridiculous, as the babies somehow find themselves in some woods in the middle of nowhere. Seasoned "Rugrats" viewers will feel a small alarm go off in their heads: something's wrong with this picture! What's wrong is that now, rather than the plotline and humor coming from the precocious babies or the silly, stupid adults, they're coming from a far too unlikely situation.

Throw in a white-water rafting sequence, a hungry wolf and a large number of runaway monkeys which have conveniently escaped from a nearby monkey-circus train wreck, and you get a movie that's a little too big for its diapers. Come on--a two-year-old who can take care of his newborn brother for a whole night, armed only with a diaper bag and his own wits? Even so, it might have worked out in the end, if the filmmakers hadn't insisted on combining "deeper issues" within the shell of this rollicking adventure.

For instance, to parallel the sibling rivalry that Tommy and Dil are dealing with, Stu and his brother Drew bring up deep-seated fraternal arguments from their childhood, only to have them neatly wrapped up at the end of the film without a word of explanation. More dramatic than this sibling rivalry theme is Tommy's dilemma: to stay loyal to his best friend Chuckie or to fulfill his responsibility to his new brother. In fact, "sponsitility" plays an almost annoyingly large part in the storyline, giving a weird allegorical bent to the movie. These rather adult topics don't merge too well with the big bad wolf and the runaway monkeys; they don't fit the silly, epic quality of The Rugrats Movie.

Indeed, the funniest parts (for the older audience, at least) occur in the beginning of the movie, before Dil is ever born. For example, at the baby shower, Tommy's aunt airily tells the expectant mother, "You know what they say: born under Venus, look for a--" before being neatly cut off by her cell phone. And Drew, when his daughter demands to know why she is being left at her cousin Tommy's house for the day, sweetly explains that "Daddy has to work overtime so Mommy won't be ashamed of his corporate earnings."

All in all, die-hard "Rugrats" fans will leave the theater a bit disappointed. The subtle humor and cute simplicity of the 11-minute plotlines on the T.V. show are for the most part absent from the movie. But it's still fun to watch-plus, it contains never-before-seen footage of Tommy's bare bottom! Who could resist?