Jason Shepherd (Frankie Muniz) is a 14-year-old boy who has lied so much in his life that even his parents have stopped trusting him. While on his way to school after writing a story for an important English class assignment that he entitles "Big Fat Liar," he is hit on his bike by a limo carrying hot shot Hollywood producer Marty Wolf (Paul Giamatti). Giving him a ride to school in fear of being sued, Jason accidentally leaves his story in Marty's car.

Switch forward to the summer, Jason is appalled to discover that "Big Fat Liar" is being made into a big-budget action film. With no one believing that he wrote the story except loyal best friend, Kaylee (Amanda Bynes), they set out for Los Angeles together determined to set the record straight with his parents and teacher by getting Marty to confess he stole the idea. What they don't expect is that Marty is a rotten, heartless human being who is not going to let his illegal actions be exposed without a fight.

"Big Fat Liar" is a delightfully witty and always entertaining comedy that uses the Universal backlots to its full advantage as Jason and Kaylee encounter myriad film sets in action and many props from past movies, such as "E.T.," "Jurassic Park," and "How the Grinch Stole Christmas." The movie is also thoroughly ludicrous, as the things the kids do in an effort to stop Marty are, at the very least, grounds for criminal arrest. We are even led to believe that they stumble into a prop room that would be any young teen's dream and move into it for a weekend without being caught. Since the picture is set somewhat in the realms of a fantasy, there is more room for forgiveness on the overall preposterous story details.

Young Frankie Muniz and Amanda Bynes are far better in their respective roles of Jason and Kaylee than they have any right to be. Both are assured and winning performers who get to show off their polished comedic skills on multiple occasions. Bynes is especially funny, whether she is calling a studio receptionist in the voice of a southern redneck, or posing as the receptionist, answering phone calls from someone named Sandler, and a "Mr. Soderbergh."

As the spiteful Marty Wolf, Paul Giamatti (2000's "Duets") also has fun with his part, appearing throughout half the movie with blue skin--a result of one of Jason and Kaylee's revenge schemes. After being sorely underused in 2001's "The Majestic," Amanda Detmer (2001's "Saving Silverman") is back in another sharp supporting turn, this time as Marty's long-suffering assistant. And Jaleel White (TV's "Family Matters") is a good sport, humorously playing himself and mocking his popularity from playing Steve Urkel.

"Big Fat Liar" ends on a feel-good note that, like much of the film, has to do with one of Jason and Kaylee's convoluted schemes that could never happen in real life, but is enjoyable to watch play out all the same. There are parts where director Shawn Levy tends toward extraneous music video montages, and does go overboard in cueing the syrupy orchestra just to elicit some forced emotions, but they thankfully do not last long enough to make a radical impact. "Big Fat Liar" is no earth-shattering accomplishment, but it is an unusually smart movie that the whole family can freely go to without a single person getting bored or talked down to.