During the pouring rain, a young boy helps his mother and father leave a heavily chained toy box at the doorstep of Foster's Home for Imaginary Friends with instructions to not be opened. The next day, Frankie has grown outraged about her job as the house caretaker. Mr. Herriman has been giving her endless chores to take care day after day, without thanking her for any of it. When Frankie discovers the box left on the doorstep, Herriman instructs her to leave it in the attic. In a fit of anger toward Herriman, she intentionally ignores the instructions given in the box and peeks inside it. She falls inside, only to discover it is a vast world filled with anthropomorphic toys and delectable treats. She hears and sympathizes with a young boy's voice (Max Burkholder), later revealed to be the imaginary friend named World (given the name by the crew of the show), who has been living alone in the toy chest since his family left the box at Foster's.Frankie adores the world and goes to visit it every day, being treated like royalty by the voice. But one day, when she goes to leave, the friend refuses to allow her and locks every exit in the castle they are abiding in. Bloo, Mac, Coco, Eduardo, and Wilt become curious with her sudden disappearance. They go up to the attic and enter the toy box, discovering for themselves the vast world that resides in it. They ask around a small town if anyone has seen a woman of Frankie's description but nobody responds. A group of weeble policemen discover they have entered the world and chase after them, but the gang are saved by a heroic man. He tries to warn them that their pursuit of Frankie will lead through extremely dangerous environments, but they are determined to rescue her. Mr. Herriman, in the meantime, is outraged by Frankie's disappearance and attempts to find a replacement for her, but to no avail.After failing to cross a musical and colorful bridge, the gang falls into a pit where sticky material becomes their zombie-like doppelgängers and moves to chase them. They escape through a Super Mario World-like environment and go to the house of a toy dog, where they are set up for a trap to eat crumpets with sleeping powder. Mac does not eat the crumpets (since the sleeping powder was thought to be powdered sugar, and he can not eat sugar without temporarily losing his mind) and is able to save the others. As they try to escape, they discover that the weeble policeman, heroic man, and toy dog are all controlled by a single face-World-who can animate and control seemingly anything he latches onto. World is trapped on an apple and the gang leaves it at a desert; but latches on a horse, truly the guise of Mac and the gang rides by, and World is able to move once more. It gallops off to the castle, where the gang find Frankie and attempt to save her. However, Frankie reveals that World never kidnapped her, she was staying of her own free will and is happy to be away from the work at Foster's and Mr. Herriman's constant nagging. The friends then plead with Frankie to come home insisting that they need her to take care of them. She believes their pleas to be selfish and storms off as she is "sick of taking care of everyone and everything and never getting a word of thanks, an ounce of help, or a tiny little smidgen of respect". They attempt to console her, but World gasses them and they fall asleep. When they awaken, they find themselves in a fake version of Foster's created by World, who shrank them into it.Frankie begins to hear their tiny voices calling to her and, though World tries to distract her from it, she finally discovers her friends. World finally becomes upset and scolds Frankie of planning to leave him alone in the toy box forever. She calms him down enough to befriend and unshrink the gang. Out of the blue, Mr. Herriman bursts into the room, having gotten into the toy box from being bugged from the Foster's residents to fill in for Frankie and do her duties. Herriman then harshly scolds World, telling him that he is preparing to take the gang home and leave him in the toy box, hoping he will learn his lesson. World's world falls to pieces as he pursues the gang until it is nothing but a white void. After running, World becomes angry and unstable and turns into a chimera-like creature to attack them all (made of the remaining refuse of his world). The gang manages to escape the toy box. Frankie climbs out as well and tries to convince everyone to let World out of the box. Herriman finally yields and releases World from the box, due to a realization of his long misjudgement of Frankie and newfound appreciation for her during her absence. World gets happy already and shouts that he's free and everyone is really confused; Frankie's answer is: "Well, think of it this way: imagine if you were able to have anything you wanted, except one thing, but that one thing is what you wanted more than anything else. For him, that thing is a friend. That's all he wanted; that's what he was trying to protect. So I brought him here. Here he can have all the friends in the world! I mean come on, isn't friendship what Foster's is all about?" Instantaneously Bloo shouts, "No, it's all about me!"; promptly Herriman slaps him with a glove, to the delight of everyone else.World adapts to the new environment and lives as a stuffed rag doll in the home. Herriman, having learned a valuable lesson from the experience, creates a new Fair Chore Act to divide the chores between the imaginary friends and thus give Frankie a well-deserved break from her job. All the imaginary friends in the house are free to travel in and out of the toy box, where they enjoy themselves (they go to the toy box in a style similar to the show's theme), with the last to enter being World and Frankie, the latter of who shout out "BOO-YAH!", ending the special.During the post-credits scene, Madame Foster returns from her vacation only to be greeted by an empty house and wonders where everyone is.