The movie follows Jason Shepherd, a teenager who is notorious for lying and exaggerating the truth. One day, he skips school to work on his English paper, which he is struggling to complete. However, on his way to school the next day, he realizes he has lost his paper. Desperate to avoid punishment, he concocts a story about a big Hollywood producer, Marty Wolf, stealing his paper and turning it into a movie.
Unfortunately, no one believes him and he is forced to stay after school until he turns in his paper. However, coincidentally, Marty Wolf has received his paper and is impressed by it. He decides to use it as the basis for his next movie, titled "Big Fat Liar," without giving any credit or compensation to Jason.
When Jason watches the movie, he is furious. He sets out on a cross-country adventure with his best friend, Kaylee, to Los Angeles to confront Marty Wolf and prove that he was the true author of the story. Along the way, they encounter various obstacles, including a stolen car and getting stranded in the desert, but they eventually make it to Wolf's studio.
With the help of some high-tech gadgets, Jason and Kaylee are able to break into the studio and steal footage that proves that he is the true author of the story. They confront Wolf and demand that he gives them credit and compensation for the movie. In the end, the truth is exposed, and Jason becomes a national hero, while Wolf's career suffers a major setback.
