The Sign of the Black Dagger — Joan Lingard

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?