The lost-and-found box in a kindergarten playground is home to Lou, a creature made from the assorted unclaimed items (whose name is made up of three letters missing from the sign on the box). After each recess period, Lou picks up the toys and items kids have left behind, leaving them in the box and subtly encouraging their owners to find them when they return.
A bully named J.J. starts stealing toys from the other children and putting them in his backpack. Lou is angered by this and decides to take J.J.'s backpack once J.J is the last kid on the playground. J.J catches him in the act and a chase ensues, with Lou constantly changing his shape to avoid being caught.
During the chase, Lou notices the name tag on J.J.'s underwear matches the name tag on a toy in the bottom of the box, an old plushie dog that a bigger kid had stolen from J.J. some years before. Lou shows the dog to J.J, but refuses to give it to him until J.J, not only returns the toys he stole but also helps find the owners of every single item in the box. J.J. initially returns the toys begrudgingly, but is surprised when a girl gratefully hugs him. He begins to enjoy returning the remaining items, making some new friends in the process.
Returning to the box for the last time, J.J. finds that Lou is no longer present, as all of his parts have been claimed by their owners. J.J. sees his plushie dog is the last toy left and happily reclaims it. A football lands near him and he decides to join two other boys for a game of catch.