When 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'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, family secrets and long-buried treasure.