Ryan, Winnie the Pooh, Ash, Michelle and the rest of their friends meet Jason Shepherd (Frankie Muniz) is a 14-year old pathological liar and con artist, deceiving and misleading his way out of trouble. After Jason fails to complete a creative writing assignment for class his teacher gives him a day to write it, or flunk and go to summer school. Struggling at first, Jason writes a story entitled Big Fat Liar, inspired by how he lies throughout his life. On his way to school Jason has a run in with arrogant Hollywood producer Marty Wolf (Paul Giamatti), whom he convinces to give him a ride. During the ride, Wolf reveals to Jason that he is also a liar and con man, but more professional. Jason accidentally forgets his report in Wolf's limousine. Wolf initially attempts to give it back to Jason, but when he sees that it is excellent, he decides to keep it for himself. Jason realizes his paper is missing, and neither his parents or teacher believe his story. Later, Jason and his friend Kaylee, (Amanda Bynes) find out that Wolf has plagiarized his paper, turning it into a film. Jason and Kaylee use their savings to fly to Los Angeles to confront Wolf. Upon their arrival in Los Angeles, they trick limo driver Frank Jackson (Donald Faison) into giving them a ride to Wolf's studio, where Jason tricks receptionist Astrid Barker (Rebecca Corry) into leaving her post to allow him to speak with Wolf. Wolf destroys Jason's paper and has the pair removed from his office. Angered, Jason and Kaylee plan to inconvenience Wolf until he admits to having stolen Big Fat Liar. They are helped by Frank who becomes sympathetic toward them after learning their goals. Jason and Kaylee sabotage Wolf: dyeing his skin blue and his hair orange, sending him to a birthday party where he is mistaken for a clown, and modifying the controls to his car. As a result of these pranks, Wolf misses his appointment with his boss and president of Universal Studios, Marcus Duncan (Russell Hornsby). Wolf and Duncan meet together at a party to celebrate the premiere of another fictional film Whittaker and Fowl, which turns out to be another box office bomb. After this failure Duncan distrusts Wolf to make another movie and threatens to pull production for Big Fat Liar unless he can convince him otherwise. Jason approaches Wolf and agrees to help him in exchange for confessing to having stolen the story. Guided by Jason, Wolf makes a successful presentation which convinces Shandra Duncan (Chris Ott) to green-light Big Fat Liar but warns him should any little mishap occur, funding for the film will be withdrawn and his career will be over. However, Wolf betrays Jason again and has Jason and Kaylee banished from his office for the second time, but this time, both their schemes and hiding place location have been uncovered and revealed as well. Wolf's assistant, having grown tired of his abuse, decides to help the pair expose Wolf. Jason and Kaylee rally together with all of Wolf's abused employees and devise a plan to stop him. As Wolf heads to the studio to begin shooting his film, his employees cause him to be late through multiple mishaps. When Wolf finally arrives on the lot, he witness Jason kidnap his stuffed monkey, Mr. Funny-Bones. After a chase across the studio, Wolf is tricked into confessing that he stole Jason's story and Duncan fires him. Jason thanks Wolf for teaching him the importance of telling the truth. He escapes an enraged Wolf by jumping off the building and onto a stunt mat. With the truth revealed Jason finally re-establishes his trust with his parents. In the epilogue, Big Fat Liar is later reproduced, utilizing the talents and skills of all those whom Wolf had abused, and released in theaters, with Ryan, Winnie the Pooh, Ash, Michelle, the rest of their friends and Jason credited as the writer. Meanwhile, Wolf declares bankruptcy and begins his new job as a clown, where he is beaten up by the birthday boy, the son of a man he had earlier insulted.
- Bobby, Cindy, Oliver, Matthew, Dwayne, Steve, DJ, Stephanie, Michelle, Malcolm, Resse, Dewey, Mallory, Renee, Daisy, Gabriella, Gerald, May, Sid, Kady, Franklin and The Gang, Alvin, Simon, Theodore, Brittany, Jeanette, Elanor, Ben Hilton, Jamie, Max, Becca, Nicky, Jeannie, The Flash/Tony, Crispin, Dylan, Duncan, Sean, Tony's Brother, German Speaking Boy, Juel, Erin, Benny, Lina, Henry, Kiki, Dennis The Menace & Friends, Pete & Pete, Richelle, Littlefoot, Cera, Ducky, Petrie, Spike, SpongeBob SquarePants, Patrick Star, Bloom, Stella, Flora, Musa, Tecna, Layla, Roxy, Simba, Timon, Pumbaa, Zazu, Terk, Tantor, Baloo, Bagheera, King Louie, the Vultures (Buzzie, Flaps, Ziggy, and Dizzy), Jiminy Cricket, Alex, Marty, Melman, Gloria, the Penguins of Madagascar (Skipper, Kolwalski, Rico, and Private), Genie, Louis, Roger Rabbit, Danny, Sawyer, Rex, Dweeb, Woog, Esla, Rapunzel, Pascal, Flynn Rider, Maximus, Jean-Bob, Speed, Puffin, Chanticleer, Wilbur the Albatross, Owen, Lindsay, Beth, Strawberry Shortcake and her friends, Danny Phantom, Sam Manson, Tucker Foley, Jazz Fenton, Danny, Sawyer, Jean-Bob, Speed, Buddy, Tiny, Puffin, Rex, Dweeb, Woog, Elsa, Rapunzel, Pascal, Flynn Rider, Maximus, Chanticleer, Wilbur the Albatross, Bob the Tomato, Larry the Cucumber, Junior Asparagus, Hercules, Megara, Philoctetes, Pegasus, Alice, Pocahontas, Meeko, Flit, Princess Aurora, Prince Phillip, Peter Pan, Tinkerbell, Rosetta, Silvermist, Idressa, Fawn, Oliver, Dodger, Lucky, Cadpig, Rolly, Spot the Chicken, Woody, Buzz Lightyear, Jessie, Tommy Pickles, Chuckie Finster, Simon, Theodore, Brittany, Huckle, Lowly, Woody Woodpecker, Ord, Cassie, Arthur Read Ash and his friends, Ronald McDonald and his friends, Bob the Tomato, Larry the Cucumber, Junior Asparagus, Laura Carrot, Jimmy and Jerry Gourd, Mr. Nezzer, Mr. Lunt, the Powerpuff Girls (Blossom, Bubbles, and Buttercup), Professor Utonium, Miss Keane, Blue the Puppy, Ploka Dots, Slippery Soap, Tickety Tock, Mr. Salt, Mrs. Pepper, Paprika, Periwinkle, Roar E. Saurus, Fredrica the Birthday Girl, Zazu, Rafiki, Kovu, Kiara, Terk, Tantor, the All-Grown Up Gang, Grandpa Lou Pickles, Carly Shay, Sam Puckett, Freddie Benson, Spencer Shay, Gibby, The Gummi Bears, Scrooge McDuck, Huey, Dewey, Louie, Webby, Launchpad McQuack, Darkwing Duck, Gosalyn Mallard, Honker Muddlefoot, Chip and Dale, Gadget Hackwrench, Monterey Jack, Zipper, Lilo, Stitch, Pleakey, Jumba, the Muppets (Kermit the Frog, Miss Piggy, Fozzie Bear, Gonzo, Bean Bunny, Rowlf the Dog, Scooter, Rizzo the Rat, Waldorf and Statler, Pepe the King Prawn, Sweetums, Thog, Sam the Eagle, the Electric Mayhem and Walter), Popeye, Olive Oyl, Popeye's Nephews (Peepeye, Poopeye, Pipeye, and Pupeye), Wimpy, Mario, Luigi, Princess Toadstool, Toad, Yoshi, Donkey Kong, Diddy Kong, Sonic the Hedgehog, Miles "Tails" Prower, Knuckles the Echidna, Amy Rose, Cream the Rabbit, Cheese the Chao, Shadow the Hedgehog, Rouge the Bat, Team Chaotix (Vector the Crocodile, Espio the Chameleon, and Charmy the Bee), Silver the Hedgehog, Blaze the Cat, Spyro the Dragon, Sparx the Dragonfly, Banjo and Kazooie, The Griffins (Peter, Lois, Chris, Meg, Brian, and Stewie), Glenn Quagmire, Cleveland Brown, The Teenage Mutant Ninja Turtles, Harry Potter, Hermoine Granger, Ron Weasley, Anastasia, Alex, Marty, Melman, Gloria, the Penguins of Madagascar (Skipper, Kowalski, Rico, and Private), King Julien, Maurice, Mort, Vitaly, Gia, Stefano, Marlene, Baloo, Bagheera, King Louie, Kit Cloudkicker, Rebecca Cunningham, the Vultures, Pinocchio, Jiminy Cricket, Geppetto, Woody, Buzz Lightyear, Jessie, Hamm, Slinky Dog, Rex, Mr. Potato Head, Mrs. Potato Head, the Squeeze Toy Aliens, Bullseye, Dolly, Mr. Pricklepants, Buttercup, Trixie, Chuckles the Clown, the Incredibles (Bob, Helen, Dash, Violet, Jack-Jack, and Frozone), Cinderella, Snow White, Ariel, Sebastian, Belle, Lumiere, Cogsworth, Mrs. Potts, Chip the Cup, Aladdin, Princess Jasmine, Genie, Abu, Mulan, Mushu, Cri-Kee, Pocahontas, Meeko, Flit, Princess Aurora, Princess Tiana, Louis, Rapunzel, Flynn Rider, Pascal, Maximus, Princess Merida, Peter Pan, Tinker Bell, Terence, Fawn, Rosetta, Silvermist, Iridessa, Vidia, Lucky, Cadpig, Rolly, Spot the Chicken, Alice, Hercules, Megara, Phlioctetes, Pegasus, Phineas and Ferb, Candace Flynn, Isabella Garcia-Shapiro, Stacy Hirano, Buford, Baljeet, Perry the Platypus, Kids World's Adventures Team, and the characters will guest star in this film.