Young Milo is bored with his life, especially going to school with its useless information. One day he arrives home to discover a mysterious tollbooth waiting in his room. In addition, there are coins and a map and so he does what any bored little boy would do under such conditions: he gets into his red car, pays the toll and drives through the tollbooth straight into a very strange world quite unlike his own.
Arriving in the Land of Expectations and consulting the map, he heads for the city of Dictionopolis. He stops to ask for directions from the Whether Man, who gives him a response is that there is no such thing as the right way and certainly no wrong roads to anywhere. Shortly afterward he is stuck in the lethargic land of the Doldrums but encounters an enormous and very shaggy dog with a large clock in the center of his body who barks loudly at anyone he finds wasting time. Milo asks the dog, who introduces himself as Tock even though he makes a ticking sound, for help in putting an end to his boredom. Tock tells him to help himself but climbs into the car alongside Milo.
When they reach the gates to Dictionopolis, the guard inquires as to their reason for requesting entry. Milo confesses that he does not really have an answer to that question, whereupon the guard pins a button on him reading “Why Not?” In Dictionopolis Milo and Tock explore the vast marketplace where words are bought and sold. Milo meets the Humbug, a large bug who is boastful but kind. He and Tock are mistakenly thrown into jail where they meet Faintly Macabre, a which who tells them about the rivalry between the king of Dictionopolis, Azaz, and his brother the Mathemagician of Digitopolis. They once got along well enough but their conflict over words vs. numbers escalated until even their lovely and wise sisters, the Princesses Rhyme and Reason, could not help. The princesses were banished to the Castle in the Air and the cities are filled with discord. Milo decides he wants to help and after he gets out of jail he tells King Azaz he will rescue them. Azaz allows him to undertake this journey, first stopping at Digitopolis to get the Mathemagician’s permission. He also tells Milo and Tock that the Humbug will come on this journey as well.
Milo’s journey to rescue the princesses introduces him to a number of strange and bizarre characters. He meets Dr. Dischord and the DYNNE who share a fascination with horrific sounds; a boy named Alec who grows downward to the ground instead of up; Chroma the Great, whose massive orchestra conducts the colors of nature; and the Soundkeeper who took all sound out of the Valley of Sound. Milo and his friends finally arrive in Digitopolis, where the Mathemagician shows them how miners dig up numbers. When Milo asks to rescue the princesses, he has to use a bit of logical trickery of the Mathemagician in order gain the necessary permission required to complete the rescue.
Milo, Tock, and the Humbug head on foot though the Mountains of Ignorance to the Castle in the Air. They ware waylaid several times by cruel and insidious demons and pests such as the Everpresent Wordsnatcher, the Terrible Trivium, and the demon of insecurity. They finally make it to the princesses and rescue them after a harrowing escape and assistance from the friends they’ve made along the way. The demons are banished and Rhyme and Reason return to the Kingdom of Wisdom.
Milo learns a valuable lesson from Rhyme that he had never stopped to consider before: the real secret to defeating boredom is not finding something to do, but finding something to do from you can learn something new. Sad to leave, but armed with newfound power of knowledge, Milo says goodbye and heads off. When he passes through the tollbooth and arrives back home, he is shocked to learn that he was only gone for a few hours since weeks had passed during his adventures in Lands Beyond.
Although he plans to go back to the magical land the next day when he gets out of school, he discovers the tollbooth is no longer there. He is initially upset but then realizes that he doesn’t need the tollbooth, as exciting adventures are just waiting to be discovered all around him.
