Youthful in two Christie books written in the 1920s, middle-aged in a World-War II spy novel, Tommy and Tuppence were unusual in that they aged according to real time, unlike Hercule Poirot or Miss Marple, whose age remained more or less the same from their first novels in the 1920s, to their last novels in the 1970s. The title of the book comes from Act 4, Scene 1 of William Shakespeare's Macbeth, when the second witch says: By the pricking of my thumbs, Something wicked this way comes.