Dorothy Gale lives with her dog Toto on a Kansas farm belonging to her Aunt Em and Uncle Henry. One day, Toto bites neighbor Miss Almira Gulch on the leg, leading her to obtain an order from the sheriff to euthanize him. In spite of Dorothy's pleas and Aunt Em's resistance, Miss Gulch takes Toto away in a basket, but Toto escapes and returns to Dorothy; she decides to run away in order to ensure that Toto won't be euthanized. Not far from the farm, she meets Professor Marvel, a kind fortune teller who uses his crystal ball to make Dorothy believe that Aunt Em may be dying of a broken heart. Horrified, Dorothy rushes home as a storm approaches; a tornado forms, and Aunt Em, Uncle Henry, and the farmhands take shelter in the storm cellar as Dorothy arrives home. Unable to be heard begging for entry, Dorothy seeks shelter in her bedroom. The window is blown in from its frame and hits Dorothy on the head, knocking her unconscious. The house is sent spinning into the air, and she awakens to see various figures fly by, including Miss Gulch on her bicycle, who transforms into a witch on a broomstick.
The house lands in Munchkinland in the Land of Oz. Glinda the Good Witch of the North and the Munchkins welcome her as a heroine, as the falling house has killed the Wicked Witch of the East. Her sister, the Wicked Witch of the West, arrives to claim her ruby slippers, but Glinda transfers them onto Dorothy's feet first. Enraged, the Wicked Witch of the West swears revenge on Dorothy and vanishes. Glinda tells Dorothy to keep the slippers on and follow the yellow brick road to the Emerald City, where she can ask the Wizard of Oz to help her return home. On her journey, Dorothy meets the Scarecrow, who wants a brain, the Tin Woodman, who desires a heart, and the Cowardly Lion, who needs courage. Dorothy invites them to accompany her to Emerald City, where they can also ask the Wizard for help. Despite the Witch's attempts to stop them, they reach the Emerald City and are eventually allowed to see the Wizard, who appears as a ghostly head surrounded by fire and smoke. He agrees to grant their wishes if they prove their worth by bringing him the Witch's broomstick.
As the foursome and Toto make their way to the Witch's castle, the Witch captures Dorothy and plots to kill her and retrieve the slippers. Toto escapes and leads her three friends to the castle. They ambush three guards, don their uniforms and free Dorothy. The Witch and her guards chase and surround them. The Witch sets fire to the Scarecrow, causing Dorothy to toss a bucket of water, inadvertently splashing the Witch, who melts away; the guards rejoice and give Dorothy her broomstick.
Upon their return to the Emerald City, the Wizard stalls in fulfilling his promises until Toto pulls back a curtain and exposes the "Wizard" as a middle-aged man operating machinery and speaking into a microphone. Admitting to being a humbug, he insists that he is "a good man but a bad wizard." The Wizard then gives the Scarecrow a diploma, the Lion a medal and the Tin Man a ticking heart-shaped watch, helping them see that the attributes they sought were already within them. He then offers to take Dorothy and Toto home in his hot air balloon, revealing that he is also from Kansas and was originally a carnival worker before a tornado brought him to the Emerald City, whereupon he accepted the job as Wizard due to hard times.
