Dorothy Gale is gladly joining her Uncle Henry in California to visit relatives who live at Hugson's Ranch, after their vacation from Australia in Ozma of Oz. Dorothy meets Hugson's nephew who is 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 a violent earthquake strikes. A crevice opens in the ground beneath them and they fall deep into the Earth.
Dorothy, Eureka, Jim, Zeb, and the buggy land in the underground Land of the Mangaboos, a race of vegetable people who grow on vines. The Mangaboos accuse them of causing the earthquake, which has damaged many of their glass buildings. Just as they are about to be sentenced to death by the Mangaboos, a hot air balloon descends, and in the basket is the former Wizard of Oz, whom Dorothy last saw as he floated away into the sky from the Emerald City at the end of the earlier book The Wizard of Oz.
