The film tells a story about a 14-year-old pathological liar, Jason Shepherd (Muniz), whose creative writing assignment is stolen by an arrogant Hollywood producer, Marty Wolf (Giamatti), who later plans to use it to make the fictional film of the same name. It was released in the United States on February 8, 2002.
Jason Shepherd is a 14-year-old chronic liar who is constantly deceiving and misleading his way out of trouble. He tries to get out of his creative writing essay by making up a lie, but eventually gets caught by his English teacher, Ms. Caldwell, who alerts his parents. He is given three hours to submit his essay, otherwise he will fail English and go to summer school. On his way to turn it in after writing his essay which are based on all of the lies throughout his life, he is struck by the limousine of Hollywood producer Marty Wolf, who gives him a ride to the community college. Along the way, when Jason admits that he is a liar, Marty admit that he, too, is a liar, but a more professional one than Jason. Jason leaves with a "word of advice" from Marty: "The truth is overrated." In a rush, Jason accidentally forgets his essay in the limo; Marty initially tries to return it, but is inspired by the story when he reads it and he decides to keep it for himself.
Jason realizes his essay is missing and tries to explain what happened, but neither his parents nor Ms. Caldwell believe him, and he is sent to summer school to repeat English. Later, he and his best friend Kaylee see a preview for a film produced by Wolf Pictures titled "Big Fat Liar", realizing it was plagiarized from Jason's essay. Determined to convince his parents he was being truthful after losing their trust, Jason and Kaylee fly to Los Angeles to confront Marty. Upon arrival, they trick limo driver and struggling actor Frank Jackson into giving them a ride to the Wolf Pictures studio. Jason sneaks into Marty's office, hoping to convince him to tell his parents what really happened, but Marty burns the essay and has Jason and Kaylee thrown out. Angered, the two decide to inconvenience him until he admits the truth, with Jackson joining them due to his own troubled history with Marty. After gathering information about Marty's cruel and abusive treatment of his employees, they begin to sabotage him through various pranks, such as dying his skin blue and hair orange, super gluing his headset, sending him to a child's birthday party where he is mistaken for the hired clown, and modifying the controls to his car, which is destroyed by a wrestler known as "The Masher".
These pranks make Marty miss his appointment with his boss, Universal Pictures president Marcus Duncan. After Marty's previous film proves to be a critical and commercial failure, Marcus loses confidence in him and threatens to pull production for Big Fat Liar. Jason approaches Marty and agrees to help in exchange for his confession. Guided by Jason, he makes a successful presentation which convinces Marcus's wife, Shandra, to green-light Big Fat Liar, but Marcus warns Marty any mistakes will cause Universal to pull funding and end his career. Marty, however, betrays Jason again and calls security to arrest him and Kaylee. Marty's assistant, Monty Kirkham, decides to help Jason and Kaylee expose him, having grown tired of his repeated verbal abuse. They rally all of his employees and devise a plan to stop him once and for all.
The next morning, as Marty heads to the studio to begin shooting, many of his employees make him be late through multiple mishaps. As Marty finally arrives at the studio, he witnesses Jason kidnapping his stuffed monkey, Mr. Funny Bones. After a chase across the studio, Marty catches Jason on a rooftop and prematurely and arrogantly celebrates his victory. He mocks Jason and tells him that he will never admit the truth to anyone, confessing that he stole Jason's story in the process. However, the conversation is revealed to have been caught on camera and is witnessed by many people including Marty's employees, the news media, Jason's parents, and Marcus. Disgusted by his plagiarism and dishonesty, Marcus immediately fires Marty, while Jason reunites with his parents, who finally learn that Jason was telling the truth, allowing Jason to regain their trust.
In the aftermath, Universal produces Big Fat Liar, utilizing the talents and skills of all those whom Marty had abused. Frank Jackson stars as the film's lead character after being rehired by Marcus, and the film is released in theaters, with Jason being credited for having written the original story, making Jason's parents and Ms. Caldwell proud of him. Meanwhile, Marty, having been stripped of his career, declares bankruptcy and begins his new job as a birthday clown. Unfortunately, he is recognized by The Masher, who orders his son, Darren, to show Marty his "nutcracker" maneuver. Marty screams in horror as Darren charges at him and flying roundhouse kicks his testicles, causing him to groan in pain.Frankie Muniz as Jason Shepherd, a 14-year-old compulsive liar and slacker who, despite his slacker personality, is actually very intelligent and a close friend of KayleePaul Giamatti as Marty Wolf, an arrogant Hollywood producer, founder of the fictional Wolf Pictures film studio and compulsive liar who, unlike Jason, does not care how his lies affect other peopleAmanda Bynes as Kaylee, Jason's best friendDonald Faison as Frank Jackson, a limo driver and struggling actor who helps Jason and Kaylee in their mission to defeat MartyRussell Hornsby as Marc Duncan, Marty's boss and president of Universal PicturesAmanda Detmer as Monty Kirkham, Marty's long suffering assistantMichael Bryan French as Harry Shepherd, Jason's fatherChristine Tucci as Carol Shepherd, Jason's motherAlex Breckenridge as Jamie Shepherd, Jason's irresponsible older sisterSandra Oh as Ms. Phyllis Caldwell, Jason's English teacherRebecca Corry as Astrid Barker, the dog-loving receptionist at the Wolf Pictures studioJaleel White as himself – often called UrkelLee Majors as Vince, an aging, but nevertheless qualified, stunt directorSean O'Bryan as LeoAmy Hill as Jocelyn DavisJohn Cho as Dusty Wong, the directorTaran Killam as Bret Callaway, a skateboard punk who consistently bullies Jason, and also has a crush on KayleeJake Minor as AaronKyle Swann as BrettSparkle (born Rachel Glusman) as Grandma Pearl, Kaylee's senile grandmotherChris Ott as Shandra Duncan, Marc's wifeKenan Thompson, Dustin Diamond, Shawn Levy, Corinne Reilly, and Bart Myer as party guestsBrian Turk as The Masher, a wrestler and monster truck driverJohn Gatins as George, a tow truck driverDon Yesso as Rocko Malone, Marty's security boss
The film was filmed at Universal Studios Hollywood, the Flash Flood set, and Los Angeles International Airport, as well as in Glendale, Monrovia, Pasadena, and Whittier, California.
The film was released in cinemas on February 8, 2002 by Universal Pictures and was released on VHS and DVD on September 24, 2002 by Universal Studios Home Entertainment.
The film grossed $48.4 million in the U.S. and Canada and $4.6 million in other countries for a worldwide total of $53 million, against a budget of $15 million.
The film grossed $11.6 million in its opening weekend, finishing in second at the box office behind Collateral Damage ($15.1 million).
Big Fat Liar received mixed reviews from critics. On Rotten Tomatoes, the film has an approval rating of 42% based on 92 reviews with an average rating of 4.9/10. The site's critical consensus reads, "Though there's nothing that offensive about Big Fat Liar, it is filled with Hollywood cliches and cartoonish slapstick, making it strictly for kids." On Metacritic, the film has a score of 36 out of 100 based on 24 critics, indicating "generally unfavorable reviews". Audiences polled by CinemaScore gave the film an average grade of "A–" on an A+ to F scale.
Some critics praised the film as energetic and witty; others called it dull and formulaic. On the positive side, Ebert and Roeper gave it "Two Thumbs Up". Critic David Palmer gave it a 7/10, stating that it is a fun one for people who love the behind the scenes of making movies, and "not awful considering it's a kids film".
A remake of Big Fat Liar began filming in August 2016. The film, titled Bigger Fatter Liar, starred Ricky Garcia as Kevin Shepherd, Jodelle Ferland and Barry Bostwick and was released on DVD in April 2017.