Halloween is coming and it's time to pick out a costume to wear. Soon Rosie has three costumes she wants to wear, but Caillou still hasn't decided on what he wants to be. And time is running out! Finally a piece of blue fake fur gives him a great idea - he'll be a blue monster! His parents help him fashion an odd but effective monster costume out of bits and pieces they find around the house. When Leo and Clementine arrive they agree that Caillou is a very scary-looking monster indeed!