When infamous escapologist Jack Door breaks out from Pentonville Prison, he sets out for the town of Brackenbridge, determined to find his missing treasure - the Moonlocket.
But when Lily and Robert unwittingly find themselves caught up in Jack Door's search, they discover that Robert's history holds the secret to the Moonlocket's whereabouts . . .
Determined to solve the mystery, Lily, Robert and Malkin the mechanical fox, set out for London on an adventure full of danger, daring, family secrets and long-buried treasure . . .