Mo is much more of a nonsense book than Oz, bringing to mind Lewis Carroll's Alice's Adventures in Wonderland, which is probably what the original title referred to.
Each chapter is a different story, unlike Baum's other books, which are full length stories. They do, however have a general throughline, and can be seen as an episodic novel.
Chapter One has no plot, but rather is a basic description of the Land of Mo, or "The Beautiful Valley". It explains that everyone in Mo is happy, and that the people never need to work, because everything they could desire grows on the trees, including items such as clothes. In A New Wonderland, the author mentions planning to move there himself, but this was omitted from subsequent editions.
Chapter Two: The Monarch of Mo goes to fight the Purple Dragon, which has just eaten all of the caramels in the land. The Dragon bites off his head, and the King is forced to go home headless. The King tries to make the best of it, but the Queen complains that she cannot kiss him anymore, so he issues an edict saying that whoever can make him a new head will get to marry one of the princesses. After two failures, a durable head is made out of wood by a wood-chopper. The Purple Dragon finds the wood-chopper and bites his head off, replacing it with the King's head. When the wood-chopper appears in court, he switches heads with the King, so that the King has his own head again and the wood-chopper has a wooden head he made. The King then tries to fulfill his promise, but the princesses refuse to marry a wooden-headed man. The wood-chopper then confronts the Purple Dragon, who tries its head-biting technique again, only to get its teeth stuck in the wooden head, thus letting the wood-chopper get his own head back so he can marry a princess.
Chapter Three: The Monarch meets a dog, who is a curiosity because there are no dogs in Mo. However, his majesty loses his temper and ends up kicking the dog who literally gets bent out of shape until he resumes his natural form again.
Chapter Four: Prince Zingle, the oldest Prince, is upset because the King will not let him milk the Ice-Cream Cow. Urged by the Purple Dragon, Zingle pushes his father down a large hole so he will become the King. The Monarch escapes from the hole and punishes Zingle by abandoning him on the Fruit Cake Island on the Rootbeer River, an island made of fruit cake. After a while, Prince Zingle gets such a furious stomachache from eating nothing but fruit cake that he repents.
Chapter Five: The King celebrates his birthday (which he does several times a year) by throwing a huge celebration, during which he entertains everyone with items from a magical casket. Everyone goes ice-skating on a lake of sugar-syrup. The sugar cracks and Princess Truella, Prince Jollikin and Nuphsed sink to the bottom. The King gets them out by fishing for them, baiting the line with a kiss for Truella and a laugh for Jollikin. But when it comes to getting Nuphsed, no one knows what he likes best, so they consult the Wise Donkey. The Wise Donkey suggests that they use an apple, knowing that it won't work. When it doesn't work, the Wise Donkey eats the apple and tells them to use a kind word. They do, and it works.
Chapter Six: King Scowleyow, who lives in a nearby country, hates the people of Mo, and has his people build a giant man out of cast-iron, designed to destroy Mo. They wind up the Cast-iron and he walks towards Mo, but trips on the dog. Prince Thinkabit figures out how to get rid of the Cast-iron Man: he tickles the Cast-iron man to get him on his back, then he pushes a pin in the Cast-iron Man to get him to stand up again, but now the Cast-iron Man is facing the other way, so he goes to King Scowleyow's kingdom and destroys it instead. The Cast-iron man eventually gets stuck in the mud at the bottom of the ocean and is never heard from again.
Chapter Seven: A boy named Timtom falls in love with Princess Pattycake, the most beautiful princess, who unfortunately has a bad temper and tries to beat anyone who talks to her. He journeys to see the Sorceress Maëtta to get her help, and along the way, he meets three animals, who agree to help him in return for gifts from Maëtta. Timtom gets a pill for getting rid of Pattycake's temper and the gifts for the animals, but they are stolen by a Sly Fox. Timtom manages to recover the gifts, thus pleasing the animals. He then goes to Pattycake and feeds her the pill. She loses her temper and then agrees to marry him.
Chapter Eight: A horrible monster called a gigaboo comes to Mo and starts destroying things. Prince Jollikin fights the gigaboo, and has his head, arms and legs cut off. Prince Jollikin manages to put himself back together, although at first he could only find his legs and head. He then saves the day by killing the gigaboo.
Chapter Nine: There is an evil wizard in Mo who is a midget and very sensitive about his height, so he tries to make a potion to increase his height. One of the ingredients of the potion is the big toe of a princess, so he steals the toe from Princess Truella. Truella gives chase, overcoming the obstacles the Wizard throws at her, and eventually kills the Wizard and recovers her toe.
Chapter Ten: The Duchess Bredenbutta falls asleep on her boat while it floats down the Rootbeer River, and so she gets too close to the waterfall at the end of the river and falls down. She ends up in Turvyland, where everything is opposite of the way it should be. With some help from a local named Upsydoun, she manages to get back to her home.
Chapter Eleven: The King's animal crackers, which are real animals, fight amongst each other, putting the King in a bad mood, so when Prince Fiddlecumdoo asks to leave Mo, the King consents, although it is a bad idea. Prince Fiddlecumdoo leaves and meets a friendly giant named Hartilaf. Hartilaf's wife accidentally runs the prince through a clothes-wringer, and Prince Fiddlecumdoo returns home, completely flat. They use an air pump to get him back to normal.
Chapter Twelve: Prince Zingle builds a large kite, which flies into the air, taking Zingle with it, eventually landing in the Land of the Civilized Monkeys, where monkeys act like humans. The monkeys do not speak English (but rather, they speak Monkey) and have never seen a human before, so they think Zingle is a dangerous animal and lock him in the zoo, where all of the monkeys come to see him, including two professors who believe that Zingle may be the missing link. Prince Zingle manages to escape and get back home.
Chapter Thirteen: The King's plum-pudding has been stolen, so he asks his wise men who did it. The wise men blame the fox, who is captured. The fox explains that he did not do it, as he was busy curing his family's sore throats by taking out the throats and turning them inside-out, then drying them in the sun. The wise men then blame the bullfrog, who is also captured. The bullfrog explains that he did not do it, as he and his wife were busy trying to save their tadpoles, who were eaten by a large fish. The wise men then blame the Yellow Hen, who is also captured. She explains that she did not do it, as her last batch of eggs accidentally produced a Hawk, not a chicken, and the Hawk took her away to a different country, and she spent the last nine days returning to Mo. The King, furious at the wise men for being wrong three times, has them put into a meat-grinder, so that they are mixed into one wise man, who tells the King that the Purple Dragon stole the plum-pudding.
Chapter Fourteen: The King holds a council of war to try to figure out how to destroy the Purple Dragon. They decide that the dragon cannot be destroyed, but at least they could rip out its teeth and make it harmless. They build a giant pair of forceps and clamp it to one of the Purple Dragon's teeth. The Purple Dragon winds its tail around a pillar to avoid being pulled by the people. As it turns out, his tooth cannot be removed, even though the men run to the other side of the valley; instead, the Purple Dragon is stretched all the way across the valley, so that it is no thicker than a fiddle-string. Prince Fiddlecumdoo cuts the Purple Dragon into fiddle-strings, and so the Valley of Mo is freed from its worst enemy.