In order to power the city, monsters have to scare children so that they scream. However, the children are toxic to the monsters, and after a child gets through, 2 monsters realize things may not be what they think.
 
FUN FACT:  In the first scene, one of the toys on the boy's bedroom shelf is the toy plane that hung from the ceiling and caused Buzz to "fly." As Mike and Sulley go through the scare floor, the rolling clown from Toy Story (1995) can be seen in the background. Lastly, when Randall is practicing camouflaging into backgrounds of walls, one of the images given to him is the wallpaper Andy had in his room.