The Scarecrow, tiny Trot from California, and Benny (a living stone statue from Boston) encounter High Boy, whose telescopic legs and airy personality make him in every way the Giant Horse of Oz! High Boy himself is on an expedition with Herby the Medicine Man and little Prince Philador of the spectacular Ozure Isles, whose home has been threatened with utter destruction by a terrifying monster sea serpent.

They all come together in a thrilling episode of Ozian history—which includes the discovery of the enchanted past of the wonderful Good Witch of the North, the very first person Dorothy met when she arrived from Kansas many years before.