The primary character in The Phantom Tollbooth, Milo, is meant to represent the typical bored child. Milo has everything a child should want iin terms of toys and entertainment, yet he is horribly, unchangeably bored. He finds everything in his life to be completely uninteresting and has a special disdain for his schoolwork, since he thinks all of it is useless. Milo does not believe that anything he learns—numbers, words, or anything else—is applicable to everyday life.

When an anonymous friend sends Milo a huge tollbooth, he begins a journey that will change his life forever. What begins as a make-believe game (and not a very interesting one, from Milo's perspective) eventually becomes a very real journey into a wonderful fantasy world called the Lands Beyond. Structurally, Milo's journey constitutes a series of lessons from the inhabitants of this unusual land. Oddly enough, Milo is never frightened by his sudden movement between words, perhaps just another indication of how truly bored he was before the tollbooth arrived.

Through his encounters with characters in the Lands Beyond, Milo learns about imagination, using his time wisely, perspective, words, sounds, numbers and a host of other things. His ultimate goal is to find wisdom—both figuratively, through his education, and literally, by locating the missing princesses Rhyme and Reason. These themes often overlap and repeat themselves as Milo finds himself in similarly sticky situations.

Milo finds himself under the wing of many characters who impart their different lessons—sometimes intentionally and sometimes unwittingly. Milo is open and attentive to these lessons, largely because he must learn them in order to escape a situation. Throughout the book, Milo learns not only values but also how to put those values to work for himself.

When he finally returns to the real world, Milo is forever changed. He realizes that he does not need the tollbooth to travel to exotic and magical places; he only needs to look around him. The once perpetually bored Milo is suddenly inspired and enthralled by practically everything. Although he is now aware of how to travel to the Lands Beyond, he decides instead to examine the things in the more immediate world around him. Where the old Milo needed the fantasyland on the other side of the tollbooth in order to be inspired or engaged, the new Milo needs only the tools he has found through the course of the book: imagination, insight, education, and wisdom.