In the world behind our closet doors, monsters like Mike and Sulley work hard for their income. To be exact, the monster world depends on our children's screams: It is their energy that makes lightbulbs glow and cars drive. Big companies like Monsters, Inc. collect the scream energy, and many monsters work there in shifts. In these times, it is getting harder and harder to shock the kids properly, since they're so spoiled by television, so that there's an energy shortage in the monster world. Sulley, the number one frightener, one day accidentally lets a human child into the monster world. Since kids are supposed to be poisonous and carry loads of diseases, pandemonium ensues. After Mike and Sulley discover that the girl they named Boo actually seems quite harmless, they decide to bring her back through her door into her room. But Boo's presence is more than just a mere accident. Now, Mike and Sulley have to face an enemy within their own ranks.