Transforming his appearance and stealing secret documents from the French is all in a day’s work for fourteen-year-old Modo, a British secret agent. But his latest mission - to uncover the underwater mystery of something called the Ictíneo - seems impossible. There are rumors of a sea monster and a fish as big as a ship. French spies are after it, and Mr. Socrates, Modo’s master, wants to find it first. Modo and his fellow secret agent, Octavia, begin their mission in New York City, then take a steamship across the North Atlantic. During the voyage, Modo uncovers an astounding secret.
The mysterious Mr. Socrates rescues Modo, a child in a traveling freak show. Modo is a hunchback with an amazing ability to transform his appearance, and Mr. Socrates raises him in isolation as an agent for the Permanent Association, a spy agency behind Brittania's efforts to rule the empire. At 14, Modo is left on the streets of London to fend for himself. When he encounters Octavia Milkweed, another Association agent, the two uncover a plot by the Clockword Guild behind the murders of important men.