This is the story of a 14-year-old boy named Jason Shepherd who lies for the fun of it. He leaves an important essay entitled "Big Fat Liar" in movie producer Marty Wolf's limo, which he then turns into a film. When Jason sees a movie preview of it he and his best friend, Kaylee, travel to Los Angeles to make Marty confess to using it to clear his name and get him out of having to attend summer school. Jason then has to match wits with Marty, who also turns out to be a big liar.