The play's story is a rather complex amalgamation of many of Seuss' most famous books. The basic plot is the same as Horton Hears a Who, although in Seussical Who-ville features characters and senarios from many other Seuss books, including the Butter Side Uppers / Downers from the The Butter Battle, and Jo-Jo from Oh the Thinks You Can Think. The Cat in the Hat also makes an apperance as the narrator.
Other books whose characters and / or settings are incorporated into the play include: