Part 1 of a 2 part story (For Part 2, see 4101): Mumford is doing his new big/little trick for everyone. While he meant to turn a big letter "M" into a little letter "M," he accidentally turns Big Bird into little Big Bird! Big Bird realizes that he is in a brand new world being little. Everything is so big and he meets small creatures he never would have been able to before. All of a sudden, he hears Snuffy calling him for their playdate. Big Bird tries to call back, but Snuffy can't hear him because he is too small. This becomes very frustrating and Big Bird wants to be big again. He figures out a plan and embarks on his trip to Alan and Gordon, who can help him find Mumford.