Crud is an evil slime monster living under Christopher Robin's bed, presumably created from all the dirt and grime from Christopher Robin not cleaning his room or vacuuming under the bed. He wants to rule the whole world by making it dirty and wants to use Christopher Robin to do this because he believes that without him putting his belongings under the bed, Crud's kingdom would not have been possible. He has a sidekick named Smudge who is always sneezing and praising Crud with names like "the master of mud" and "the surveyor of slime." He also has crayon soldiers and used them to kidnap Christopher Robin's friends. He even uses some of Christopher's toys as his slaves. Despite being a Winnie the Pooh character, Crud is dark and malevolent. He intends to make the world more dirty by spreading his dirt and grime everywhere with his reverse vacuum, and loses his temper if anyone talks about cleaning and washing, even claiming that he hates the words "clean" and "washed up," as he hates being cleaned up. With his messy slimy exterior and his reverse vacuum cleaner, he plans Christopher Robin to help him make the world dirty.
When Christopher Robin and his friends throw things under the bed, Crud is able to transform it into a world and kingdom of its own. Pooh and Piglet go through first, and are captured and imprisoned by Christopher Robin's crayons, turned into Crud's guards. Later, Crud's minion, Smudge, leads Christopher Robin and Tigger to Crud, who tells Christopher Robin that his kingdom was created because of him throwing things under the bed. When Christopher says he should clean up under the bed, Crud becomes enraged. He forces Christopher Robin's toys to pour slime into his "un-vacuum," a reverse vacuum cleaner designed to spread dirt and grime all over the world. Crud tells Christopher Robin to help him with this, under a threat to presumably kill his friends, but he refuses, so Crud has him imprisoned. However, he and the others escape before Crud confronts them again. Christopher exposes Crud to a bar of soap, weakening him, and then the toys modify the reverse vacuum into an actual vacuum cleaner. Together, they and Tigger suck Crud and Smudge into the vacuum, thus "cleaning them up" and erasing them from under the bed. To prevent the creation of another monster like Crud, Christopher Robin promises his mother to always clean his room afterwards.