When Will and Lucy's dad disappears, their life changes for ever. They uncover a trail of secrets and lies - but hidden in a fireplace in their old Edinburgh house, there's a clue that might lead them to the truth. A journal that tells the story of another pair of twins, William and Louisa, who lived in their house two-hundred years before. Their own father was in serious financial trouble and took sanctuary - as all debtors could - in the grounds of Holyrood Abbey. A place full of French aristocrats fleeing revolution; a haven of intrigue and spies. In 1796, a mysterious black dagger helped the children save the life of an exiled French Count. And now it will lead Will and Lucy to find their own dad - and the truth about why he's disappeared.