By Pricking of My Thumbs: Agatha Christie

An old woman in a nursing home speaks of a child buried behind the fireplace...When Tommy and Tuppence visited an elderly aunt in her gothic nursing home, they thought nothing of her mistrust of the doctors; after all, Ada was a very difficult old lady. But when Mrs Lockett mentioned a poisoned mushroom stew and Mrs Lancaster talked about `something behind the fireplace`, Tommy and Tuppence found themselves caught up in an unexpected adventure involving possible black magic...