Milo, a bored, lonely boy who lives in a San Francisco apartment block, is surprised by the sudden arrival of a large, gift-wrapped package. Inside is a tollbooth, which turns out to be a gateway into a magical parallel universe. As Milo passes through the tollbooth, the character moves from live action to animation, and his toy car transports him to the enchanted Kingdom of Wisdom and the cities of Digitopolis and Dictionopolis.
Accompanied by a watchdog called Tock (because he has a large clock face embedded in his body), Milo has a series of adventures in places like the Mountains of Ignorance, the Doldrums, Dictionopolis, Digitopolis, and the Castle in the Air. Together they must rescue the Princesses Rhyme and Reason, who are being held captive in Castle in the Air, and restore order to the Kingdom of Wisdom. The many eccentric characters they meet include the noisy Dr. Dischord, the Mathemagician, King Azaz, the Senses Taker, and Officer Short Shrift.