London 1938. Fifteen-year-old Arabella Swain and her cousin Humphrey are spending a long, hot summer under the watchful eye of their housekeeper Mrs McKay, completely unaware of the adventure and danger which await them.

Thousands of miles away, in the desert mountains, a young corporal and his captain find themselves swept up in a treacherous sandstorm. The corporal stumbles across a cave, where he finds not only shelter but a secret passageway, marking the start of a chain of events which lead him to London and Arabella’s home.

At the heart of the mystery is a statue of a tiger, discovered by the teenagers’ grandfather; a famous archaeologist, and housed in the British Museum. The statue is linked to an ancient civilisation, and a long-lost book: together, these treasures prove to be a powerful force and the race is on to find the book as a sinister secret order seeks to steal the statue and harness its powers to take control of London.

With her mother in Mesopotamia on the hunt for the book it is up to Arabella, along with Humphrey, their friend Tom and the indomitable Mrs McKay, to unravel the mystery and foil the evil plan. Bravery and cunning are key - but so is knowing who they can trust.