Emerald City of Oz

At the beginning of this story, it is made quite clear that Dorothy Gale (the primary protagonist of many of the previous Oz books), is in the habit of freely speaking of her many adventures in the Land of Oz to her only living relatives, her Aunt Em and Uncle Henry. Neither of them believes a word of her stories, but consider her a dreamer, as her dead mother had been. She is undeterred.
Later, it is revealed that the destruction of their farmhouse by the cyclone back in The Wonderful Wizard of Oz has left Uncle Henry in terrible debt. In order to pay it, he has taken out a mortgage on his farm. If he cannot repay his creditors, they will seize the farm, thus leaving Henry and his family homeless. He is not too afraid for himself, but both he and his wife, Aunt Em, fear very much for their niece's future. Upon learning this, Dorothy quickly arranges with Princess Ozma to let her bring her guardians to Oz where they will be happier and forever safe. Using the Magic Belt (a tool captured from the jealous Nome King Roquat), Ozma transports them to her throne room. They are given rooms to live in and luxuries to enjoy, including a vast and complex wardrobe, and meet with many of Dorothy's old friends, including the Cowardly Lion and Billina the Yellow Hen.
In the underground Nome Kingdom, the Nome King Roquat is plotting to conquer the Land of Oz and recover his magic belt, which Dorothy took from him in Ozma of Oz. General Blug suggested that King Roquat have their forces dig a tunnel under the Deadly Desert. After ordering the expulsion of General Blug (who will not agree to such an attack due to the powers of Princess Ozma) and the death of Colonel Crinkle (who also refuses) where he was sliced thin in a torture chamber and fed to a bunch of Seven-Headed Dogs, King Roquat holds counsel with a veteran soldier called Guph. Guph believes that against the many magicians of Oz (the reputation of which has grown in the telling), the Nome Army has no chance alone. He therefore sets out personally to recruit allies from other parts of Nonestica.
Dorothy, accompanied by the Wizard of Oz and several other friends, departs the Emerald City in a carriage drawn by the Wooden Sawhorse, intending to give her aunt and uncle a tour of the land.
The Nome General Guph visits three nations: the Whimsies, the Growleywogs and the diabolical Phanfasms of Phantastico. Having learned of this through Ozma's omniscient Magic Picture, the people of Oz become worried. As the Nomes dig a tunnel for the combined armies to get under the Deadly Desert to the heart of the Emerald City, Ozma uses her Magic Belt to wish for a large amount of dust to appear in the tunnel. Upon emerging, the Nome King's allies therefore drink thirstily from the nearby Fountain of Oblivion, whose waters make them forget their evil plans. The Nome King himself avoids the drink, but is thrown into the fountain by the Scarecrow and the Tin Woodman, which erases his memory too.
Ozma uses the Magic Belt to send the Nome King and his allies back to their respective lands. To forestall a future invasion of Oz, Glinda the Good Witch uses a magic charm to render Oz invisible and unreachable to everyone except those within the land itself.