By the pricking of my thumbs - Agatha Christie

When Tommy and Tuppence go to visit elderly Aunt Ada in her nursing home, they think nothing of her mistrust of the doctors; after all, Ada is a very difficult old lady.
But when another resident, Mrs Lancaster, starts talking to Tuppence about there being something behind the fireplace, Tuppence is strangely disturbed.
And when they return to Sunny Ridge to find that Mrs Lancaster has gone, Tommy and Tuppence find themselves caught up in a spine-chilling adventure which could spell death for either of them...