On a small farm in the great Kansas plains a young girl named Dorothy Gale lives with her Aunt Em and Uncle Henry along with her dog and only friend, Toto. One day an enormous cyclone hits the farmhouse. Aunt Em and Uncle Henry are able to hide in the storm cellar, but before Dorothy and Toto can reach the cellar the cyclone picks up Dorothy's house with her and Toto inside of it. The house lands in a beautiful world where Dorothy is greeted by three short men in blue clothing and a short old woman wearing a white gown and pointed hat.
The old woman tells Dorothy she is now in the Land of Oz and claims to be the Good Witch of the North of the Gillikin Country. The witch tells Dorothy that when she arrived her house landed on and killed the Wicked Witch of the East, the vicious and now former ruler of the Munchkin Country. The only thing that remains of the witch are her magical silver shoes which are given to Dorothy by the Good Witch of the North. Dorothy tells the witch that she wishes to return home to Kansas, so the witch tells Dorothy of a powerful wizard who rules over the Emerald City in the center of Oz. She gives her a magical kiss on the forehead which will protect her and sends her down a road of yellow bricks which leads to the Emerald City.
Along the way Dorothy comes across a corn field and meets a talking scarecrow who befriends Dorothy and comes with her on her quest to the Emerald City in hopes of the wizard giving him a brain. Eventually the two come upon a man made entirely out of tin who has become completely rusted. Dorothy and the Scarecrow find an oil can and use it to restore the movement of the Tin Woodsman who tells them that he was once in love with a woman and planned to marry her, but the old woman she was a servant for did not want her to leave, so she paid the Wicked Witch of the East to prevent him from marrying her; which she did by enchanting his axe so he would continue to chop off parts of his body. Every time he chopped off a body part a tinsmith would help him by replacing the body part with tin. He eventually chopped off all of his body parts and became completely made of tin, but when the tinsmith made his body parts he forgot to put in a heart for him and without a heart he could no longer love the girl he used to be in love with. Dorothy and the Scarecrow invite the Tin Woodsman along the way, so he can receive a heart from the wizard.
The three of them enter a spooky woods where they are attacked by a ferocious lion. They soon discover the lion is nothing but cowardly and lacks courage. The lion joins the group seeking courage from the wizard. The four of them face kalidahs, creatures with the head of a tiger and the body of a bear. However, they are managed to escape, but soon come across a field of magical poppies which puts, Dorothy, the Cowardly Lion, and Toto fast asleep. Since the Scarecrow and Tin Woodsman are not made of flesh they are able to carry Dorothy and Toto across the poppy fields, but the Cowardly Lion is too heavy for them to lift. The Tin Woodsman seeks help from a talking mouse known as the Queen of the Field Mice. The queen and her subjects help them by carrying the lion across the poppy fields ending his slumber.
The four finally reach the beautiful, green Emerald City where they are given a pair of green spectacles at the gate and are told by the gatekeeper they must wear them in order to protect their eyes from being blinded by the dazzling sights of the city. The four each meet with the wizard in his throne room individually who appears to them in four different forms; he appears to Dorothy as a giant, floating head, he appears to the Scarecrow as a beautiful, winged woman, he appears to the Tin Woodsman as a gigantic lizard creature, and he appears to the Cowardly Lion as a blazing ball of fire. They each state their requests and the wizard tells each of them he shall not grant their wishes unless they destroy the Wicked Witch of the West who rules over the Winkie Country.
The witch saw the four of them coming to destroy her so she sent her armies after them to no avail; first she blew her enchanted silver whistle once and summoned a group of wolves whose heads where chopped off by the Tin Woodsman, then she blew the whistle twice and summoned an army of angry crows whose necks were strangled by the Scarecrow, then she blew the whistle three times and summoned a swarm of black bees that all died after stinging the Tin Woodsman, finally she blew the whistle four times and summoned her Winkie slaves who were scared away from Dorothy and her friends by the Cowardly Lion. The witch finally resorted to using her magical Golden Cap which could only be used three times to summon her army of Winged Monkeys who tore up the Scarecrow, threw the Tin Woodsman against sharp rocks, and captured Dorothy, Toto, and the Cowardly Lion. The witch made Dorothy her slave while she starved the lion until he agreed to be her slave as well. The witch wanted Dorothy's silver shoes and was able to use trickery to steal one. An angry Dorothy threw a bucket of water on the witch which melted her in the process, allowing Dorothy to recover her stolen shoe. The citizens of the Winkie Country rejoiced after being freed from the witch's tyranny and helped Dorothy restore the Scarecrow and the Tin Woodsman and made the Tin Woodsman their new ruler.
Dorothy took the witch's Golden Cap and used the cap to command the Winged Monkeys. Her first request was for them to fly her and her friends back to the Emerald City. After arriving in the Emerald City Dorothy and her friends discovered that the wizard was not a wizard at all, but nothing but a short old man hiding behind a curtain. He explained to them that he was once was a magician from Omaha who was travelling in a hot air balloon with his first two initials, O.Z., on the balloon, when the balloon got caught in a vicious storm and crashed into the Land of Oz. Since the citizens of Oz did not know of hot air balloons, they all assumed he was a wizard because he came from the sky and their country's name was on the balloon. The people of Oz built a great big city for him and proclaimed him as their ruler. He did not want to hurt anyone's feelings, so he used machinery in order to fool everyone into thinking he was a wizard and made everyone wear green glasses so the city would appear to be green when in reality it was not. Dorothy and her friends were furious at first and were disappointed that they would not receive their wishes. However, the wizard explained to the Scarecrow, Tin Woodsman, and Cowardly Lion that they already had what they sought. The three were still not happy, so as placebos he placed a bag full of bran, needles, and pins inside the Scarecrow's head as a brain for him, he placed a phony heart made of velvet and sawdust inside the Tin Woodsman's stomach as a heart for him, and he made the lion drink a green liquid as courage for him. Their wishes are granted except for Dorothy's who still wishes to return home. The wizard repairs his hot air balloon in order to take Dorothy back to Kansas and gives his throne to the Scarecrow proclaiming him the new ruler of Oz. The balloon however sets off too early, taking the wizard and leaving Dorothy in Oz. Dorothy uses the Golden Cap a second time to ask the monkeys to return her home, but she is told they cannot cross the desert that surrounds Oz. She is told by a solider with green whiskers to seek help from Glinda, the Good Witch of the South who rules over the Quadling Country.
The four venture into the Quadling Country where the Cowardly Lion defeats a giant spider that was terrorizing a forest's animals, so they proclaim him the King of the Beasts. Along the way the four encounter Fighting Trees, a country made entirely out of china, and armless creatures known as Hammer-Heads. Dorothy uses her final wish and asks the Winged Monkeys to carry her and her friends over the mountains. Dorothy finally meets with Glinda who tells her that she always had the power to go home all this time and that the Silver Shoes can take her anywhere she wishes to go. Glinda takes the Golden Cap from Dorothy and commands the Winged Monkeys to take Dorothy's friends to their new given kingdoms, orders them never to bother others, and gives the cap to the King of the Winged Monkeys, thus freeing them of its spell. Dorothy bids goodbye to her friends and clicks her heals three times as she and Toto are transported back to Kansas with Aunt Em and Uncle Henry.
- The Wicked Witch of the West is the half-sister of the Evil Queen from "Snow White", and daughter of Cora (who is the miller's daughter from "Rumpelstiltskin", and the Queen of Hearts from Alice's Adventures in Wonderland).
- The Wicked Witch is green-skinned, rides a broomstick, and often dresses in black with a pointy black hat as in the 1939 movie adaptation by MGM.
- The Wicked Witch apparently gained her green skin texture out of intense jealousy while she was in the Enchanted Forest for the first time.
- The magic shoes are silver as they are in the book, but are simply called slippers.
- It was the Wicked Witch, not Dorothy, that exposed the Wizard of Oz's true persona. She turned him into one of her Winged Monkeys.
- The Flying Monkeys are people transformed by the Wicked Witch into her pets rather than being a race of reluctant servants controlled by a magic cap.
- The Witch of the South, Glinda, is the keeper of the Great Book of Records; a fact introduced in The Emerald City of Oz, one of L. Frank Baum's sequels to The Wonderful Wizard of Oz. In Once Upon a Time's adaptation, the book tells the future of Oz as well as the past and present whereas in the novels it only records things as they happen.
- The Wicked Witch falsely gives the impression she melts as a result of Dorothy throwing water on her.
- The Cowardly Lion, Tin Woodsman and Scarecrow are not present, although the Wicked Witch seeks a heart, a brain and courage as ingredients for a time travel spell.
- The Witch of the East is a good witch and is not wicked.
- Dorothy's house did not kill the Witch of the East, and landed in the West instead of the East.
- Dorothy arrived in Oz alone without her dog Toto.
- The Wicked Witch of the West, while disguised as the Wizard of Oz, gives Dorothy the silver slippers to send her back to Kansas.
- Dorothy Gale
- Glinda, the Good Witch of the South
- Munchkins (mentioned)
- The Wizard
- The Wicked Witch of the West
- Winged Monkeys
- The Good Witch of the North
- The Wicked Witch of the East