The movie is about a little twelve-year-old girl who's family decides to move. Most of the movie follows the characters inside her head, each one representing a major emotion like "Joy" or "Sadness." The movie centers around teaching kids why they are feeling what they are feeling when upsetting life things take place. The movie even ends with a surprisingly mature message that memories don't have to always be "happy" or "sad." As we get older they are sometimes a mixture of the two.
What actually impressed me the most about this movie was how much they explore the concept of memory formation. Obviously, it's dumbed down for the sake of the movie. But there were a surprising number of general concepts addressed. The best part (even though it's a bit sad in the movie) is when the character Joy ends up getting lost in the land of unused memories. The movie delves into why they are unused and why they must be dumped in order to make room for new memories. "Cleaning house" as it were.
Since seeing the movie I've brought up this scene with several of my students. Each time they instantly understood what I meant by memories (using the movie as an example) and then it was easy to segue into how practicing helps to keep certain memories vs. having them become unused. The movie allowed them to visually see the effects of practicing.
So, if you're a teacher, the movie is definitely worth a watch.