By the Pricking of My Thumbs (Tommy & Tuppence Mysteries)

When Tommy and Tuppence visit an elderly aunt in her gothic nursing home, they think nothing of her mistrust of the doctors; after all, Ada is a very difficult old lady. But when Mrs. Lockett mentions a poisoned mushroom stew and Mrs. Lancaster talks about "something behind the fireplace," Tommy and Tuppence find themselves caught up in a spine-chilling adventure that could spell death for either of them....