"Seussical" is a musical that brings together characters from various Dr. Seuss stories. The main plot revolves around Horton the Elephant, who discovers a speck of dust with a tiny world called Whoville on it. He dedicates himself to protecting the Whos and their leader, the Mayor of Whoville.
Yertle the Turtle appears briefly in the musical as a villain who is trying to conquer the other turtles, but he is not a central figure in the story.