Edie Burchill and her mother, Meredith, have never been close, but when a long-lost letter arrives one Sunday afternoon with the return address of Milderhurst Castle, Kent, Edie begins to suspect that her mother's emotional distance masks an old secret.

As a thirteen-year-old World War Two evacuee, Meredith was sent to live with the mysterious Blythe family: Juniper, her twin sisters Percy and Saffy, and their father, Raymond, author of the 1918 children's classic The True History of the Mud Man. In the grand and glorious Milderhurst Castle, a new world opened up for Meredith. She discovered the joys of books and fantasy and writing, but also, ultimately, their dangers.

Fifty years later, as Edie chases the answers to her mother's riddle, she, too, is drawn to Kent and the eccentric Sisters Blythe. Inside the decaying castle, Edie begins to unravel Meredith's past; but there are other secrets hidden in the stones of Milderhurst, and Edie is about to learn more than she expected.