Leo is not having fun! His head teacher is in the living room with his parents for a severe look at Leo's school record. While Leo is in his room, trying to look his best for the occasion and getting ready to go down, Mot arrives with some strange colored marbles, which Leo takes to be chewing gum and immediately swallows. The result is devastating. Leo goes through every stage of rubbery metamorphose, and is going to have a hard time allaying his parents' suspicion and finding a way out of the situation. Not to mention the fact that his hand is roving about the neighbor's place, that Mot is also affected by this phenomenon and that Diane, his only hope, has just swallowed a blue marble ...