Numbuh 86, ever the caretaking friend to the imaginary friends at Foster's, becomes thoroughly disgusted with being bossed around by Monty Uno day by day combined with the endless chores and no help or even a simple "thank you" for all the work she does. However, one day, a mysterious locked-and-chained-up toy box arrives at the door with a letter that explicitly states that 1) an imaginary friend is inside the box and 2) the box must NOT be opened at any point ("Imaginary Friend Inside. DO NOT OPEN!"); despite Frankie's protestations, she is ordered by Herriman to take the box up to the attic. The box is taken up to the attic, and against the instruction of both Herriman and the letter-in-tow, she decides to open the box and free the friend. She initially looks inside the unusually-deep box, only to accidentally fall into it, which she realizes is a world in itself. She also hears the childlike voice imaginary friend (named "World" by the creators of the show) from nowhere, and from that point, she explores this world-in-a-box, one that is filled with living toys, crayons, sumptuous delicacies and visual beauties, with this new friend; her friend, who has dwelt in this world alone after being shipped by its kid's family (depicted as a prologue earlier in the film) to Foster's, talks to her and sympathizes with her real-world plight. She promises to return the next day, and it soon becomes a routine for her to fulfill her tiring job and then take a dive inside the toy box to take a load off and be treated like royalty. However, one day, she is about to leave for the real world, but then the imaginary friend (who also controls everything in the world in the toy box) locks the doors and windows of the castle in which she was staying for the afternoon.
Numbuh 1 (in a sleepover with Numbuh 4 at the Home), along with Maurice, Numbuh 3, and Numbuh 2 ponder the whereabouts of Frankie, as she hasn't made her tasty French toast breakfast. A furious Mr. Herriman bounces to the attic to find Frankie, only to see the toy box's chains broken; he then bounces out, threatening to hand her a pink slip. Left in the attic, Mac and the others look for Frankie, only to find that the toy box is not only alive, but ticklish; they then tickle it to open the lid, and then fall inside as well to find Frankie.
They then wander into a town in the world, only to find that none of the toys notice or respond to them. They are then pursued by Weeble-like policemen to a river of plastic balls, into which they fall after a long battle with the police. They float down the river, but are then pulled under by a large glove-covered hand. The hand, they realize, belongs to a tall, handsome Robin Hood-esque hero figure who talks in an older form of Shakespeare-style English tongue. The Robin Hood-like figure, who yells "Excelsior!" as a battle cry, repeatedly tries to warn the group away from pursuing their quest as he follows them through different, trying environments that the group overcomes, like a forest full of springs or a sea of lava. Meanwhile, Mr. Herriman is trying to get a new employ for the job, but no avail.
After failing to cross a piano-like bridge, the group fall into a gooey, sticky canyon full clay-gunk zombie copies of themselves (When Bloo sees his zombie copies, he screams, "It's an army of zombies! Incredibly handsome zombies!"). The Robin Hood figure sacrifices himself while the group makes their escape. They eventually end up at the home of a stuffed toy dog who takes them to his home to rest for the night feigns kindness but then feeds them crumpets that are dusted with sleeping powder (which he presents to them as powdered sugar). The sleeping powder was a trap to keep them away from Frankie when tied up, but Mac pretended to eat the crumpets because he's not allowed to eat sugar, that begins to fool the toy dog by a clay gunk zombie copy of him and didn't get trapped, but then the toy dog's face (which is World's true form) slips off of the dog and onto the body of a stuffed squirrel, who then comes to life. The group realizes that the face can animate anything that he slips onto, like the leader of the Weeble policemen they saw earlier, the "Excelsior" guy, and the toy dog.
Capturing World on an apple that is held onto by Mac (he can't move to another body as long as he is held by Mac away from other objects, and Mac is off-limits to him since Mac already has a face), the group then moves the apple to a nearby desert of burning-hot sand and leaves him there. The apple dries up, but then a group of toy horses gallops by; World on the apple then calls out to a stray horse that then attempts to eat the apple, only to become World's new courier. The horse, now possessed by World, then gallops to the castle, and World slips off through several different objects until he ends up at the tallest spire where Frankie is staying; satisfied that Frankie hasn't left, he leaves for the lower area.
The horse, which is really the disguise of Mac and the group, then runs to Frankie's room, only to find her on a flat bed with a faceless muscled masseuse massaging her back. They plead to Frankie to come back to the real world, but Frankie takes their pleas as acts of selfishness, thinking they only liked her because of all the nice things she did for them. She then refuses to come back with them. The group is then gassed out cold by World, who possesses the figure of a sorcerer.
Waking up, the group finds itself back in the attic, and Mac despairs over Frankie's words while the group wanders downstairs to their rooms. However, they soon realize that they are not in the real Foster's, and Mac is made aware of this when he sees a giant green eye peering into the attic window. Running downstairs, the group is tossed around by the house as it is moved topsy-turvy; when it stabilizes, the group opens the door of the house.
At Fosters, Mr. Herriman was mopping the floor until the rest of the Imaginary friends were still upset about their problem. But before Mr. Herriman can continue, they start passing the mop around until they talk about their problem. That nagged him, and Mr. Herriman said that he was sick of all the things they did and he hopped off to the attic to find Frankie.
Frankie, who is enamored with the world that World has built (which she already knows to be the animator of toy figures), is disturbed from her observations by tiny, squeaky voices. Afraid that she may find out, World tries to coax her away from the miniature Foster's House, but a paper plane flies into her head; picking it up, Frankie finds the miniature group inside. World then becomes fiercely angry at her discovery, accusing her of having made up her mind to leave him alone in the toy box, recounting the similar situation that resulted in the toy box being sent to Foster's in the first place; Frankie rejects the accusation, calming him into unshrinking Mac, Bloo and the group and making friends with them. Suddenly, an upset Mr. Herriman hops into the room and founds them. Herriman then scolds World for bringing and keeping Frankie in the toy chest, and says that he is going to drag Frankie and the others out of the toy box and leave him alone to think about his actions.
Becoming distraught and furiously by this, the entire world crumbles as he pursues Herriman, Frankie and the rest; after a long and hard retreat, World becomes a huge dragon-like combination of the many objects of the box and easily defeats Wilt, flinging him, blows Coco away with his wings, and nearly kills Bloo by shooting him with a laser, opening a fissure that threatens to kill him but Eduardo saves him, Eduardo then confronts World and manages to force him back, both then dash towards each other but before they can clash, World cheats and flings Eduardo away with his tail, and finally he chases Mr. Herriman. After realizing that World is large enough for them to use to get back home, Mac and the others, with the exception of Frankie, get onto his back. Soon all but Frankie make it out of the toy box as World, still in his monster form, eats Frankie and swallows her whole. Frankie, however, emerges out of the box unharmed, and then pleads with the group to let World out of the box. A bitter argument ensues between Frankie and the rest, but then Mr. Herriman consents to her opening the box, cracking a very-rarely-seen smile. The group then lets her open the box, and a very frightened World (with whom she negotiated her release in return for his release) emerges. World gets happy already 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, when 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?" World soon adapts to the outside world, and Frankie knits him a stuffed body; the residents also agree to cooperatively take care of cleaning and care taking affairs for Frankie so that she can have a temporary respite from her duties, and all the residents soon join Frankie and World in jumping into the toy box world.
During the post-credits scene Henrietta Von Marizpan shows up at the door, refreshed from her vacation but wondering where everyone is.
- Tara Strong as Timmy
- Daran Norris as Cosmo
- Susanne Blakeslee as Wanda
- Ben Diskin as Numbuh 1 and Numbuh 2
- Lauren Tom as Numbuh 3
- Dee Bradley Baker as Numbuh 4
- Cree Summer as Numbuh 5
Original Motion Picture Soundtrack
- Enjoy Yourself (A+)
- Inkanyezi Nezazi (The Star & The Wiseman)-Ladysmith Black Mambazo
- Strong (Robbie Williams)
- Witch Doctor (Cartoons)
- Just Looking (Stereophonics)
- We Like To Party (The Vengabus)-The Vengaboys
- Thank Abba For The Music (Steps, Tina Cousins, Cleopatra, B*witched & Billie)
- When The Going Gets Tough The Tough Get Going (Boyzone)
- Umbrella (Rihanna feat. Jay-Z)
- Erase/Rewind (The Cardigans)
- The Sweet Escape (Gwen Stefani feat. Akon)
- Better Best Forgotten (Steps)
- Cupid's Chokehold/Breakfast in America (Gym Class Heroes)
- Believe (Cher)
- Fountains (Kate Nash)
- Tender (Blur)
- Girlfriend (Avril Lavigne)
- What Can I Do (The Corrs)
- Shine (Take That)
- Honey To The Bee (Billie)
- Do You Know (The Ping Ping Song) (Enrique Iglesias)
- You Don't Have To Say You Love Me (Dusty Springfield)
- Love Today (Mika)