Since he was six years old, Milo has lived on board the Sesquipedalian, or "Quip," a magical train that uses the power of imagination to travel through both stories and the real world. Aboard the Quip, Milo lives and works with his uncle, Horatio, a book smuggler who trades in rare books. When Horatio takes on a dangerous new job, he needs the help of Tilly Pages, a uniquely gifted bookwanderer. But when Tilly's grandfather and Horatio are poisoned by a mysterious copy of The Wizard of Oz and fall into a deep sleep, Milo and Tilly find themselves racing against time to save them. The friends must journey to the Emerald City with Dorothy, and eventually to Venice, Italy, to find out who is behind these strange poisonings.