In the story, set after the last book, Dorothy is joining Uncle Henry in California at Hugson's Ranch, on their way home from Australia, Dorothy having visited friends in San Francisco. She strikes up an acquaintance with Hugson's nephew and her second cousin, Zeb of Hugson's Ranch. Dorothy, Eureka (her cat) and Zeb are riding a buggy being pulled by a cab-horse named Jim when an earthquake starts and opens a crevice beneath them that sends them hurtling into the bowels of the Earth.

Dorothy, Eureka, Jim, Zeb, and the buggy alight in the Land of the Mangaboos, a race of vegetable people who accuse them of causing the Rain of Stones (what the Mangaboos call the earthquake because they are beneath the surface of the Earth, and earth instead falls on them). Zeb is surprised by this strange new land, but Dorothy surmises that they are in a fairy country because they are meeting vegetable people and Jim and Eureka are now speaking. Just as they are about to be sentenced to death by the Mangaboos, a hot air balloon falls out of the sky and in the basket is the Wizard of Oz (who Dorothy last met as he floated away from the Emerald City)...