One day, Will and Lucy's dad just ... vanishes.They have no idea why he's disappeared until a creepy stranger reveals their dad was keeping a BIG secret. Then there's the second clue: an old diary they find hidden in the walls of their Royal Mile house, with a sinister black dagger on it. Will and Lucy must solve a mystery that's over two hundred years old if they want to find their dad and bring him home. But can they find the answers in time to rescue him? Award-winning author Joan Lingard weaves a fast-paced mystery set in and around Edinburgh's Royal Mile. The story alternates between Will and Lucy, searching for their dad in the present day, and their ancestors William and Louisa, struggling to save their own father while following the sign of the black dagger and uncovering a plot to kill a French aristocrat.