Jason is first seen arriving to his English class late and lies to his teacher about the essay and is forced to write a real essay or go to summer school if he doesn't hand it in. After he finishes the essay which are based on all of his lies throughout his life, a villainous Hollywood producer named Marty Wolf steals his paper and turns it into a film called, Big Fat Liar, his parents and his teacher don't believe him and Jason ends up losing his parents' trust after Jason tried to explain to his parents that Wolf stole his paper and turned it into a film. He and his best friend, Kaylee head to Los Angeles to confront Marty Wolf for stealing his paper. After Wolf destroys his paper rather than giving Jason back his paper and calling his parents about the theft which infuriates Jason, he and Kaylee manage to get revenge on Wolf by turning his skin blue and his hair orange and supergluing his earpiece to his ear as well as sending him to a birthday party where he can get beaten up and re-arranging the controls to his car with the help of Wolf's limo driver and former actor, Frank Jackson as a punishment for stealing his paper and destroying his paper and Jason will give Wolf one phone call to his dad if Wolf wants to end this. Jason also made a deal with Wolf an exchange in confession of stealing the story from Jason in order to have Wolf call his dad to tell him the truth. However, he and Kaylee were forced to leave Universal Studios after Wolf calls security to have Jason and Kaylee sent home for good instead of calling Jason's dad about Wolf stealing Jason's story and Jason decided to tell his parents the truth, but Wolf's assistant, Monty Kirkham, decided to help Jason and Kaylee to stop Marty Wolf from filming. After Jason gets chased by Wolf after showing him that he has Mr. Funny Bones, Wolf confesses to everybody that he has stole Jason's paper and turned it into a film, only to have Wolf fired when he says it out loud on the camera and exposing him of his true colors. Jason manages to escape from an enraged Wolf (but not before thanking him for teaching him the value of telling the truth) and re-establishes with his parents' trust as his parents apologize to him for not believing about Jason writing about his essay and the production of Big Fat Liar continues on forward with Frank Jackson as the lead character after being rehired by Wolf's boss and the President of Universal Studios, Marcus Duncan. Towards the end of the film, the film, Big Fat Liar is released in theaters as it became an instant hit with Jason credited as the original writer of the story as Jason's parents and his teacher are proud of him.
"Yeah, from my backpack you loser!"
"See it? I think I wrote it."
"So you admit you stole my story."
"We're dealing with the meanest man alive."
"Because the truth's overrated, right?"
"The truth is not overrated."