Edie Burchill and her mother have never been close, but when a letter arrives one Sunday, marked with the return address of Milderhurst Castle, Kent, Edie begins to suspect that her mother's emotional distance masks an old secret. Inside the decaying castle, Edie begins to unravel her mother's past. But there are other secrets hidden in the stones of Milderhurst Castle, and Edie is about to learn more than she expected.