Milo is a chronically bored little boy. He can’t amuse himself or be happy anywhere. One day, after racing home from school and preparing for another boring afternoon, Milo notices a big mysterious package in his room. It’s a kit to put together a small purple tollbooth that leads to “lands beyond.” Since he has nothing better to do, Milo puts it together. He gets in his toy electric car and puts his coin in the tollbooth.
Suddenly, Milo is driving along a beautiful country road. First, he comes to a sign that instructs him to honk for advice. When he honks, a man introduces himself as the Whether Man and welcomes Milo to Expectations. Confused by the Whether Man’s odd behavior, Milo drives on. But soon after, Milo starts to daydream. As he stops paying attention, he takes a wrong turn and the car stops. A small creature on Milo’s shoulder says this is the Doldrums, where nothing changes and nothing happens. He and his fellows are Lethargians. Milo got here because he wasn’t thinking; thinking is against the law in the Doldrums. Just as Milo is about to accompany the Lethargians on holiday, a watchdog named Tock, whose body is an alarm clock, chases the Lethargians away. He tells Milo how to get out of the Doldrums (Milo must think) and asks to accompany Milo to Dictionopolis.
Finally, Milo and Tock reach the gates of Dictionopolis. Immediately upon entering, the king’s five advisors welcome Milo to town and confuse him, as they all say the same thing using different words. Milo has never paid much attention to words before this, but now they seem interesting, so he browses the words in the market. At a stall that sells individual letters, Milo meets the Spelling Bee, who encourages him to learn to spell. But then the Humbug appears. The Humbug believes spelling is overrated, so he and the Spelling Bee fight—and knock the entire market over in the process. When Dictionopolis’s one-man police force, Officer Shrift, arrives, he blames Milo for the damage. After giving Milo a short sentence of “I am,” Officer Shrift puts Milo and Tock in prison for six million years.
In the dark dungeons, Milo and Tock meet what they think is a witch. But the kind old woman, Faintly Macabre, says she’s actually a Which—she used to choose which words people could use, but when she started to hoard words and brought about silence and economic depression, the king put her in prison. She can’t get out until the princesses Rhyme and Reason return to the kingdom.
Faintly Macabre tells Milo the story of the Lands Beyond. There used to be nothing but monsters and demons in the land until a prince arrived from across the Sea of Knowledge and, over the years, established the Kingdom of Wisdom. He had two sons who founded Dictionopolis and Digitopolis, and when the king was old, he discovered two baby girls under his grape arbor. He named the girls Rhyme and Reason, and as they grew, they ruled on matters of state with fairness and compassion. Things were fine until the king died. His sons, King Azaz the Unabridged and the Mathemagician, constantly fought over whether numbers or letters were more important, and their fight reached a climax. When Rhyme and Reason ruled that letters and numbers were equally important, the new kings imprisoned them in the Castle in the Air, which is perched high above the Mountains of Ignorance. The demons still live in the castle. Milo suggests he rescue the princesses, but Faintly Macabre says it’s too difficult. But she does note that there’s a door out of the dungeon.
Immediately upon stepping into the sunshine, Milo and Tock are swept up by the king’s advisors to attend a royal banquet. There, Milo meets Azaz the Unabridged and is tasked with deciding the menu. Milo’s suggestions of “a light meal” or “a square meal” end up being terrible, as waiters bring platters of dancing light and steaming squares. Milo is upset; he didn’t realize he’d have to eat his words. Once all the guests, save the Humbug, have left the hall, Milo suggests that things might make more sense if Rhyme and Reason returned. Azaz agrees that this is a great idea. He gives Milo a gift of all the words he knows and insists the Humbug will accompany Milo and Tock on the journey.
After driving a while out of Dictionopolis, Milo and his friends come across the Point of View. There, they meet Alec Bings, a little boy who floats in the air—in his family, Alec explains, children grow down to the ground. That way, they never have to change their point of view. He leads Milo through the forest and shows him the towns of Illusion (a beautiful city that doesn’t exist) and Reality (a city that doesn’t exist anymore because people stopped noticing it, but where people still live). Then, they attend the evening concert. But rather than play music, the huge orchestra plays all the colors in the world, conducted by a man named Chroma. Once Chroma has conducted the sunset, he leaves to sleep and asks Milo to wake him so he can play the sunrise in the morning. In the morning, Milo decides conducting the sunrise himself shouldn’t be too hard—but the musicians won’t stop playing and end up playing through an entire week of color. But once the unsuspecting Chroma plays the sunrise, Alec takes his friends back to their car and gives Milo a parting gift: a telescope, so he can see how things really are.
Next, Milo, Tock, and the Humbug come across Dr. Dischord and his assistant, a cloud of blue smog called the DYNNE. Dr. Dischord creates all unpleasant noises and is thankful for the rise of big cities; now, people want noises like screeches and honks. He warns Milo that to get to Digitopolis, Milo will have to go through the terrible Valley of Sound. The valley seems normal to Milo—until suddenly, he realizes he can’t hear any noises or make noise of his own. The silent residents of the valley explain, by writing on a chalkboard, that this is because of a woman called the Soundkeeper. The people need Milo’s help: if he can bring a tiny sound out of the Soundkeeper’s fortress, they can knock it down and release sound back into the world.
The Soundkeeper, to Milo’s surprise, can speak—as can Milo while he’s in the fortress. She agrees to show Milo how she catalogues sounds and the facilities where she used to make sounds. She explains that the Valley of Sound is silent because, when Dr. Dischord “cured” everyone by causing them to only be able to hear terrible sounds, the Soundkeeper decided people couldn’t have any sound if they only wanted awful ones. Milo can’t figure out how to sneak a sound out—until he starts to object to something she says and is able to keep the “but” he was going to say on the tip of his tongue. Once he rejoins the valley’s residents, he drops the “but” into a cannon, which shoots the word into the fortress. The fortress crumbles. The Soundkeeper sobs that she regrets what she did and gives Milo a gift. It’s a package of sounds, and it includes a number of laughs.
After this, as Milo, Tock, and the Humbug drive along, they remark that things couldn’t be going better. This causes them to fly suddenly to the Island of Conclusions, where a man named Canby shares that they must swim back to shore through the Sea of Knowledge. Once back on shore, they continue to Digitopolis.
The first being to meet them is the Dodecahedron, who shows the travelers to the numbers mine where Digitopolis mines all the numbers in the world. In the mine, they also meet the Mathemagician. Then, in the Mathemagician’s workshop, the Mathemagician shows Milo the biggest and longest numbers (a giant three and a giant eight, respectively). Realizing Milo actually wants to know how big infinity is, he sends Milo up a staircase with infinite steps. On the steps Milo meets the left half of a child who loves averages—he’s part of an average family, which includes 2.58 children. He’s the .58 of a child. Disillusioned, Milo returns to the Mathemagician’s workshop and, using logic, convinces the Mathemagician to let him rescue Rhyme and Reason (the Mathemagician and Azaz have sworn to disagree on everything, so because Azaz agreed to let Milo rescue the princesses, the Mathemagician initially refuses). He then gives Milo his own magic staff (a pencil) to help him on his journey.
The Dodecahedron walks Milo, Tock, and the Humbug to the foothills of the Mountains of Ignorance, where the travelers find they have to walk. They keep a close eye out for the demons. Soon, they meet their first demon: the Terrible Trivium, who tries to stop the travelers forever with mundane tasks. Milo escapes once he uses the magic staff to figure out how long it would take to perform the tasks. He, Tock, and the Humbug hurry on—but the demons start to chase them.
Milo and his friends use the gifts he picked up earlier in his journey to outsmart the Demon of Insincerity, the Gelatinous Giant, and the Official Senses Taker. The demons are right behind them as they run up the giant spiral staircase and enter the Castle in the Air, where they meet the princesses Rhyme and Reason. The princesses assure Milo that it’s okay he took so long; he was learning, and that’s important. As the demons try to cut away the staircase and send the castle flying away into the sky, the princesses say to let the castle go—it’s just a prison, anyway.
Everyone grabs onto Tock, who can fly since “time flies.” They reach the ground and race on. Just when it seems like the demons are going to catch them, the armies of Dictionopolis and Digitopolis arrive to beat them back. With Rhyme and Reason returned, everyone celebrates. Then, it’s time for Milo to go home. Milo bids his friends goodbye, gets in his car, and drives back through the tollbooth. When he gets back to his room, he realizes he’s only been gone an hour.
The next day after school, Milo is excited to go back through the tollbooth—but when he gets home, the tollbooth is gone. Milo is sad for a few minutes, but then he realizes that he has more than enough to keep him occupied here, in his room.