Halloween is just around the corner, but Caillou doesn't yet have a Halloween costume. Seeing a bed sheet gives him an idea. He could be a ghost. Caillou explains to Rosie about Halloween costumes and Mommy suggests that Rosie could be a princess. Caillou, however, apparently isn't satisfied with the ghost idea, as he asks Daddy to help him find a costume. Daddy dresses Caillou in some grown-up clothes. It's a fun idea, but he can't walk around very well in it. There's still a few days left, though. Time passes and Caillou still doesn't have a costume. He decides to go down to the basement, where Daddy paints whiskers on him and makes him into a cat. This idea doesn't really work for Caillou, but it goes into Rosie's costume. She's now become a "princess / clown / cat." Finally, Caillou stumbles upon the perfect idea for a costume a monster. He imagines being a scary monster and picking candy from trees. With Mommy and Daddy's help, Caillou now has a great costume, which really impresses Leo (a cowboy) and Clementine (a bumblebee) and they all go trick-or-treating.